86 Camaro ignition (ECM) bug - no codes

I have an '86 Camaro with a carbuerated 305. It has developed a particularly menacing bug that I can't find. It can crank, but won't run. When I can get it to run, it runs very rough and won't idle. Here's the crazy part: when I raise the RPM above 2600 RPM, it runs quite smoothly. Drop the RPM below 2600 and it staggers and stumbles again. The ECM isn't registering any codes (perhaps because it won't run longer than a few seconds?)
I have isolated problem to either the ECM or one of its sensors. Disconnecting the distributor from the ECM will allow the engine to run, also putting up code 41 (saying that the distributor isn't connected). However, doing forces the distributor to fully retard the timing and the ECM makes the carb run rich (killing both performance and my mileage). I have also changed the pickup coil, ignition module, cap and condensor, plug wires and plugs.
I thought that I had fixed the problem when I replaced the BARO and MAP sensors. It ran fine for about 10 minutes, then died in the highway again. This problem first presented itself the same way: it was running fine, then died in the middle of the road. Disconnecting the computer altogether will get it running again. Thankfully the carb and HEI aren't THAT dependend upon the ECM. However, it's killing my fuel milage and the ignition won't advance or retard without its signals from the ECM.
Has anybody seen a similar problem where an engine will only run well above 2600 RPM based on something from the ECM or one of the sensors? The SES light works - it just doesn't give any codes saying anything is bad...
thanks much,
-rkrjr72

Just for the heck of it, can you get ahold of a vacuum gage and check your manifold vacuum. The symtoms sound like a vacuum leak or if this was a newer vehicle Mass Air Flow sensor.