possibly bad ecm?

91 sierra, 350, auto

This truck has been a very good one, (300,000 miles plus) with an engine change about 60,000 ago. Started running rough at idle recently, (faltering when under a load and almost completely stalling), and mileage dropped from 18 to 14. I have replaced plugs, wires, rotor and cap, and tps. Map and egr codes are displaying sometimes, however they both check out fine. Idle air valve has also tested good. I am thinking something may be wrong with the ecm, because when the truck has been running a while (warm), I can't get any codes at all to display, even though the ses light is on, not even the standard test code (12). When it has cooled off, I can extract some of the test sequence, but the ses light gets very dim progressively sooner the more times the test is run. Sometimes I can't even get past the test code display (code 12). Truck has plenty of power, but during acceleration (heavy loads and high engine speeds), a violent short valve clatter happens, and immediately corrects itself followed by a service engine soon light. Again, codes 32 and 33 are showing, when I can get anything at all to show. 32=egr malfunction or failure, and 33=map, or vice versa. Have any of you ever seen anything like this? Any help is appreciated.

Got a new ecm today. Fixed almost all issues with the pickup. Turns out the only real problem I have is a bad knock sensor. Gonna replace that tommorow and see if the pinging goes away.