I had the same problem with my 1985 380 SE, it also ran fine until warmed up and then ran terrible.
At cold start the engine temp sensor reads cold and keeps the computer in open loop control where the oxygen sensor is not used to adjust fuel mixture.
As soon as the temp sensor goes above a set temp 160 ?
the computer goes into closed loop operation and uses input from the o2 sensor to constantly adjust the fuel mixture and keep emmissions down.
I used this logic to replace my o2 sensor and the old girl runs great.
For the price it is the best place to start.
Hope this helps