Thanks for the info. I got some 1/4" glass from a glass supplier and that fixed all my issues. Turns out it WAS my glass flexing while being heated.

Quote Originally Posted by jasay View Post
Tightening up the temperature swing might help. You can autotune the PID settings: http://reprap.org/wiki/PID_Tuning

Also make sure the sides of the glass are not touching anything so that it can expand. I'd still try the tape/cold bed first though just to make sure this is the root cause.