Makerbot replicator 2 stops extruding

So I have had this replicator 2 for about two years now. It has printed wonderfully since I received it. Recently though it stopped printing. I load in the filament and it goes through fine. Then I will start up a print (that I have printed before so i know its not just the file) and let it go. It will successfully put down the first few layers or even half the object sometimes. Then this is when it stops. It will let out small amounts at different points where it passes over the previous layer. If I pause the print and try and push the filament through myself it does not go through. When I remove the filament the section that was inside the hot end it hard and breaks off easily. If I take off some and load it again then it will continue to push through the filament. For the most part I have always used default settings with different tweaks for certain prints but since I have encountered this problem I have changed back to basic default. I am also using Makerbot PLA. I have tried to change the heating temperatures, used different filament, replaced the nozzle, leveled the bed. This is everything I could think of. I have looked online but not seen anyone that has had the same problem I have encountered just similar ones. When I switched from makerbot PLA to deltamaker PLA I was able to successfully print a whole hand. From then on it stopped with that filament as well.

When you replaced the nozzle, did you replace the ptfe tube (in the heat-throat) as well? Often the last part of this tube plugs up with crud which leads to blockage. You will need to take off the heater block to get to it. And you need to have the right ptfe tube handy of course....

When I replaced the nozzle I made sure that there was no blockage inside the tube.

How did you determine that?

It may still have a reduced/warped last end. I have replaced several myself which seemed to be fine when pushing through them, but they kind of crumple up in the end part when heated which increases the backpressure during printing (and yes, ususally after the first few layers when the ptfe has heated up higher in the heat throat).

Also crud on that part may release particles during printing, blocking the nozzle again. When you push through the nozzle with a guitar string or drill, it seems fine as you push the particle to the side. However, when printing it just floats back to the orifice again.

What you are looking for (if this is your problem) is the last 8-10 mm or so of that ptfe tube. Take it out and have a look at it.

Another less likely option is a slipping drive gear which needs to be cleaned. You will need to remove the fan to get that one.

Is the extruder fan functioning properly? Is the extruder stepper motor getting too hot?