Have an interesting one on a 93 16v 4-door.

After the vehicle warms up and switches to low idle it drops down to between 400-600 rpm and does not want to accelerate.  Disconnecting each of the fuel injectors causes the rpm's to drop even further so it's not an injector issue.

Seems to run fine when still cold and in a high-idle state.

There is an engine light on but when you jump the wires no error code comes out.

CPU related maybe?

Hrm, the low idle makes me think EGR hanging open... but then, why only warm? But, the CEL doesn't make sense... I think the 93 has the nag switch, try flipping it to the other position & see if it will throw you a code then. Or better yet, find the switch & follow its wires up to the back of the instrument panel & disconnect it. Mine doesn't have it, but I understand its lock works in reverse of most, you have to pull the tab up, not squeeze it in to disconnect it. Then no more nag light ever, so any CEL will only be real codes.
You may be right though on the ECU issue, it is in the era of the bad caps... if they've never been replaced that could well be it. If it has the black rubycon caps I'd solder in some fresh ones regardless. C111 is the usual culprit. Hth

Fuzz