I believe is is the flexibility in the top case pushing the RAM off it's slot. The battery screw slots often break off, contributing to the flexibility. Missing or loose screws especially in the bottom case will also allow more flexing. Tighten everything, if you can't anchor the battery, shim it to keep it from moving when flexed.
I totally Mr. Myagi’ed it, by rubbing my hands together until they burned, then held the phone un my hot hands. I did this 5-6 times over a couple of minutes. It was a good zen way to get started!