I have a 1995 Bonneville SE also and had this exact same problem. It got worse over time, and I tried nearly everything you did to get it working, with no luck.

Turns out the ECM was shot and needed replacing. Then everything was fine. Keep in mind this is ODB-II connector car with ODB-I guts so the fact that you're not getting any codes, do you know for a fact that it is actually linking up or just not seeing any errors.

My shop bill totalled $383 for this including parts and labor.

i had the same problem what ended up being the problem is the wires going from the ignition module to the crank and cam sensor for some reason they we shorted out and burned from being too close the the block with no wire loom check the wires going to those and remove any wire loom and tell me what u see

when you do the code scan, do you atleast get the diagnostic mode code?