98 Frontier 2.4 Idles Surges

I have a 98 Frontier that idles at around 2000 when started and then when warms up will fall to 1500 then right back to 2000 and continuously surges between 1500 and 2000.

I have a 98 but haven't had this problem, however I did have an 87 4x4 and I got the computer wet and had to replace it with a used one. The idle was never right after that. It did much like you describled. have you replace you computer recently? That is about all I can think of.

The computer is still the original.