That definitely sounds like it isn't a GPU problem. Was that 75% user with 25% idle, 25% system, or 25% wait?
idle - CPU isn't loaded either, could be IRQ or DMA problem
system - likely driver, possibly incorrectly setup kernel (preempt can cause this)
wait - IO problem. either hard drive problem (is DMA turned on? 32 bit transfer? read ahead?), something wrong in IRQ or DMA settings for card, or driver problem.
I've used the values from htop, so if i understand great, its idle time...
But back - it really can be CPU problem, if it works correctly for some time after return from suspend?