I'm having computer, ECM, problems with by beloved 1992 Covette, LT1. (ECM 16159278)

I bought it new, it began to have ECM problems about two years ago. I had the OptiSpark unit replaced, but now ECM "appears" to be dead.

When I switch it on, both fans turn on and the display flashes sys about every 20 seconds. I can't get coolant temp or oil temp (except from guages). But it
will crank every time, when cold and then dies when the engine warms up.

On rare occasions the ECM works perfectly until the engine gets hot and then the engine dies and you have to wait a while until it cools to restart.

I built my own device to read the serial data, but I can't make heads or tails of it.

When I removed the ECM and applied power, I get nothing at all on the serial pins.