1982 Chevy Imapala ECM question

I just bought a 1982 305 Impala to use as my winter driver and as a hobby fixer car. It is in good condition but it has a few faults. It starts right up and runs good and strong. For the past few days it has had the check engine light on. I have a repair manual and it told me how to check the codes. I grounded the diagnostic pin and got no change in the steady light. After checking the fuses I found the ECM fuse was blown. I replaced the fuse and tried again. This time I got no trouble code (only code 12, meaning no pulse from distributor, verifying the ECM was working and capable of throwing error codes.). The engine idle was higher when cold with the ECM working, but seemed to run fine other than that... until the engine warms up. When the engine is warm and the ECM is on the engine runs rough anywhere between idle and full throttle. After a quick and harrowing drive with the ECM running I tried to check for codes again... Now the check engine light doesn't work at all... Any ideas. My guess is the O2 sensor because it doesn't happen when cold.

they probably disconnected the fuse because the ecm is bad. The ECM's in older cars like you didn't do much other than slight fuel/air mixture adjustments, that's why it runs w/o the ecm fuse in place.