The iMac at my shop has been experiencing frequent hangups and failed boots in the past couple weeks, to the point where I...

Hooked it up to an external display and got the same glitches. Opened it up, removed the logic board, dusted everything out, applied new thermal paste to the gpu, and it's turning on now, but still showing some static-y glitches, and is generally sluggish. Could it still be both the logic board AND the gpu?