AE92 ECU / Tunning probs

[mr_happy]

Standard AE92 Auto with a 4AGE. Seems to be firing late (doesnt idle at 500rpm when warm, will always stall, but is fine at 800, but it does go up and down by 200 rpm at that speed).

How easy is it to reset the factory computer, or will I have to muck around getting someone to tune it for me?? It does feel like its firing much later than it should be (and no, I dont think its a spark plug prob, its been doing this for yonks).