I don't know what I'm talking about and not really a typical troubleshooting thing but... as an extreme option you could try installing and running EQ from a VM. That way you could limit access to the CPU and/or take your graphics card out of the equation entirely (if you can't already do that? Should try if you can). Download Virtualbox and find a winXP or linux ISO (I'd go with Linux Mint and wine as a beginner-friendly linux option myself), fiddle around with the hardware settings, install the OS, and go from there.
Before that I would try setting these lines to FALSE in eqclient.ini, see if that helps any at least on non-P99 servers:
Code:
VertexShaders=FALSE
20PixelShaders=FALSE
14PixelShaders=FALSE
1xPixelShaders=FALSE