FPS perma-drops

Hi all,

I'm using an AMD Radeon HD 7970M laptop GPU with switchable graphics options (basically onboard). I'm experiencing a problem that only surfaced recently - after playing for a while, or sitting in a place with a large number of objects or effects, my FPS takes a permanent nosedive. It goes from 40/60 - 12/15 FPS and never reverts. Nothing I do fixes or resets this short of restarting the client.

I've searched all the answers, changed all settings, reinstalled all drivers. Turned down tesselation, turned off HDR, turned off Vsync, limited Max FPS, etc. - My GPUZ programs tell me my temp is fine, and my Resource Monitor shows no unusual use of Memory or HDD activity, so I want to throw this out there. Any suggestions?

Here's my system specs:
Windows 7 Home Premium 64-bit (6.1, Build 7601) Service Pack 1 (7601.win7sp1_gdr.150928-1507)
Intel(R) Core(TM) i7-3610QM CPU @ 2.30GHz (8 CPUs), ~2.3GHz
RAM: 12 GB (I've tried this with 8, no difference)
Main GPU: AMD Radeon HD7970M

Running on DX9.