Timing problems - timer2 and SPI (noob)

I think I've made a design mistake. I was hoping to drive a pair of cascading shift registers by using the SPI.h functionality, and have my timing dictated by using timer interrupts running at 7800 hertz or so.

I can do EITHER of these things independently: run the shift registers (in a hard-coded loop) OR use the interrupt (TIMER2 calls a test function).

But I cannot seem to make the interupt drive the shift registers.

Is there some contention between timer usage and SPI?

(Sorry, I had to re-post this because I screwed up the original description of the problem :blush:)