Shifting timing on GT4

I yanked the distributor to fix an oil leak and popped the distributor back on, taped a degree wheel on the harmonic balancer, hooked up the timing light and - what the? The car was idling fine in its usual high rpm warm up, but the strobe light indicates that timing jumps by 180 crankshaft degrees every now and then, but not just a flash here or there, it switches and holds there for a while and then goes back. What is going on? It does it front and rear banks. Both Pertronix pickups are in one dist, but other than that the systems share nothing, seperate coils, caps, etc. And it shifts by exactly 180*. Crossfiring between wires? I turned all the lights off and watched in the dark, but didn't see anything.

The second half of the video is a better representitive sample of what it looks like. Any ideas?

http://www.youtube.com/watch?v=GmgWjmiiDFs
(the beginning video is crappy, but in real time it looked like the second half of the video, must have been some camera anomoly with lighting or something)