Brand newbie - what is wrong with these prints?

[BKTarHeel]

Hi, everybody. First post!

Being a masochist, I put it in my 9-year old son's head that he should ask Santa for a 3d Printer. Santa shopped around and got a 6" MakerFarm Prusa i3. Turns out Santa is not such a speedy 3D Printer builder, and only got the kit about 3/4 of the way done before Christmas morning. Oh, well, he left a nice note telling my son that he thought we would enjoy finishing it together. (Seemed to work). Working on it with my son was fantastic, which was my present to myself. We spent days getting the printer very close to correct (Colin was great and patient, by the way).

We've spent the last two or three days doing test prints and are having trouble with anything taller than the test hollow cube. Can someone help us diagnose? I flashed the RAMPS to up the max Magma hot end temp. I also changed my print bed temp to 110/110 in Slic3r. My X-axis belt as a little loose, and I will print some tensioners from Thingiverse to fix that soon, but I don't think that is the problem.

The prints in the picture are little creeper figures my son designed in PrinterCraft. They should have a square head that narrower than the base. I arranged them from left to right, latest to earliest. They are sticking well to the glass now (getting the Z-end stop right was tricky) but at some point, they shift and I get the following result. I have a big brim on them, and and printing with support material turned on in Slic3r (I didn't remove it in the leftmost print).

One weird thing is they seem to be getting closer to finished in the last two prints without me doing anything.

Any help would be appreciated. Thanks!

IMAG0955_BURST002.jpg