Odd problem, first layers great middle bad, last great again

Ok i have been trying to print a the Bottom of the power supply so i can add a on/off switch to my Prusa Knock off. I have noticed a really odd thing. I'm doing a 500 layers, the first 150 layers are perfect, then 150 layers are brittle, much like its under extruding, pourus, then the top 150 layers are fine.. Basically the middle of the print is bad. If it was good and then got bad and stayed bad it would make some sense, but its great, crap then great again. I have printed this box 4 times and its the same every time. its like the printer can print perfect up to about 10CM, then from 10CM to 30CM its crap, then 30CM + is ok again. I have switched filament, 3 different ones used with same results so it seem its the machine. It got all the marking of poor extrusion, but why would it be ok, bad then ok again in same print? MY spool is free, no binding that i see. I am having fun, but 6 hours a print and all failing is starting to get to me :) I have change layer height, even went up 10% on flow, still same in middle of print. I know this is a hard one I see very little like this when i search.

I'd check that your software isn't doing something similar to Simplify3D (italics added):

"To do this, Simplify3D allows you to specify how many solid layers you want on the top and bottom of your part. For example, if you were printing a simple cube with 5 top and bottom solid layers, the software would print 5 completely solid layers at the top and bottom of the print, but everything else in the middle would be printed as a partially hollow layer. This technique can save a tremendous amount of plastic and time, while still creating very strong parts thanks to Simplify3D’s great infill options. However, depending on what settings you are using, you may notice that the top solid layers of your print are not completely solid. You may see gaps or holes between the extrusions that make up these solid layers. If you have encountered this issue, here are several simple settings that you can adjust to fix it."

Artly, I did check that, but thanks for the suggestion. I have been cruising Youtube and came across a filament temp video, the user was printing a tube and decreasing the temp every 20 or so layers till the print looked like it should. His prints reminded me of mine (I would have put a picture up, but I'm using transparent filament so its hard to get a good shot). After watching it I reduced my temp by 20 and it "seems" it may have solved it. 6 hour print job so wont try till tomorrow. My defaults where 200 for PLA (printer default) but it seems 180 is much stronger. I have 2 printers i just put together in the last month, a discontinued 3DFabrikator (hobby king) and my i3 clone. I couldnt figure out why the Fabrikator parts seemed flexible but my prints with the I3 where pretty hard and brittle like. I had printed a bunch of Coins and calibration cubs and they all looked great, maybe a tiny bit "hard". I think what is happening is LONG prints the print head is getting to warm, hence it works ok to start but an hour to 2 hours in it seems to get very brittle, and hard. I am hoping a reduction in temp may help, my head may still heat up but if i start lower maybe I can get by. My print head does have a FAN and the LCD shows only a 1-2 degree change when printing.. but I am wondering if its 200 is correct, maybe im really at 210? Just guessing. Maybe my temp prob is bad/off (doubtfull as it goes from room temp to 200 and back down when done but who knows). Will update if the lower temp printing helps as soon as i get a chance to test it more then some simple tall tubes.

OK found the problem for sure, how to fix not so sure. The Extruder is the issue, it prints fine, watching the feed gear, all is well then about 1-2 hours into print the feed starts retracting in tiny bits. it shouldn't be, its not skipping area's its just drawing a simple wall, it should be a steady flow, but it starts stuttering. My i3 knock off hides the gear so it was a bit tough to see. I am wondering if the filament binds would the extruder try a quick retraction then pull again to correct? I have pulled it all apart and nothing binds, only think i noticed is the top of the filament alignment block is beveled and smooth, the bottom is sharp as heck and could hang on filament if retracting, which i will bevel to solve. But my problem is not that its hanging on retractions, but that its doing it at all when it should just be extruding. I notice it not an even retraction even, the say the gear moves from 12 to 3 (clock wise) then it will retract to 1 and go to 5, then retract to 4 and maybe go all the way to 12.. its not even hicups. It was even going perfect for a round or two then the mini retractions start happening again. Thius is happening to anything i print that takes 2+ hours, its extruding great then it starts that hjicuping in the gear.. I have seen some youtubes on boards getting to hot, I wonder if that could be it. i have printed enough calibration cubes that are excellent, but hte big things (long prints) are all failing 2-3 hours in.. and now I know why, but how to fix.