My '90 Accord starts and runs great until it reaches operating temp then it hunts at idle and stutters on acceleration from a stop. Check engine light comes on intermittently. I replaced the oxygen sensor, no change. Please help as I need to pass emissions this month. Also, does this car have onboard self diagnostics??

It should have a diag port located under passenger side dash, to the right most wall. It's a 2 port connector. You can use a paper clip to short it and read the code on the check-engine light or S light.

Do an EGR test.

I have the same problem - someone suggested fuel additives. Any suggestions?

If your check engine light is coming on, fuel additives wont help...it is a problem and you need to fix it. To fix it, you need to know what is specifically wrong, and to find that out, you need to pull the codes. Check the FAQ for directions if you don't know how.