Rather than put the exhaust back on and drive, I decided to try a couple more tests. I connected a vacuum gauge and started the car and warmed it up. At idle, it read a little low and fluctuated. Indicating a retarded timing. At around 2500RPM it would read fine at times and then read a little low (same flucuation as at idle). I swapped the map sensor with a known good one...no change.

My timing light is on the fritz so I need to borrow one to verify the timing is really retarding. But in the mean time, other than the knock sensor, what else could cause the fluctuations? Again, this comes and goes, idling and driving now. When it first started it only appeared to affect the engine while driving (loss of power, burnt out converter, glowing exhaust manifold). Just wanna cover all my bases before I plunk down another $50!