1990 Integra

Car starts when cold, but not right away, and it will drive okay. Once it's warm the idle starts to surge up and down and it stumbles if I try to drive it. I get trouble code 43.

I've done some searching and found that a lot of people solve this by replacing the 02 sensor or sometimes fuel filter.

However, since my car is only doing once it's warm I'm not convinced those are the problems. I've read the computer will give code 43 if the mixture is lean/rich mixture, and since it only does it when warm am thinking it might be the IAC valve. (I checked the FITV and coolant temp sensor).

Any ideas of where I should be looking? Don't want to just start replacing parts.

Thanks!