If you are questioning the crank position (RPM) sensor, have you verified spark at the plugs - ALL 4 PLUGS (just in case there is a problem in the distributor cap or rotor).

How have you verified crank/cam timing? If you are relying on the timing mark on the outer ring of the the crank damper, don't. The outer rings are known to occasionally spin on the rubber damper between the outer ring and the hub. Also don't rely on the painted marks on the belt itself. Here is a good page on timing belt alignment:

http://www.brickboard.com/FAQ/700-900/B230FTimingBeltAlignment.htm

If you can then be confident that the damper pulley hasn't spun, you can put a timing light on plug lead #1 and crank it over to see if #1 is truly firing at TDC. (That just verifies that something else hasn't slipped or spun like the keyway on the cam sprocket or something gotten out of whack in the distributor, if that's even possible.)

Did you replace the entire exhaust system from manifold to tail pipe? I'm thinking about the possibility of a clogged cat or something. Any rags stuffed in the header pipe while you had the top end apart? Was it apart long enough that some critter could have "taken up house keeping" down in the exhaust pipe? Oh yeah - check out the air filter box and intake duct. My mother-in-law's 740 had a nest of dead baby rabbits in it. I'm just reaching for any possibilities here.