rpms going 1000-2000 in neutral

Hi,
My rpms keep jumping from 1000 to 2000 rpm and back , up and down, say when I'm at a stop sign. It happens when I'm in neutral, or when I step on the clutch. Also a problem is that when I start the car the rpms start really high, a little above 2000 and stay there forever, the lowest it goes, when it's really wormed up is 1600(that is if I don't start driving it, then it starts jumping up and down).
What could be the cause for that, none of the mechanics I know seems to be able to fix it. What should I try changing. I've already done spark plugs, and wires. I changed water pump and timing belt less than a year ago. It's a 94 integra ls, with a little over 150 000 miles.
Thanks so much in advance, please help me out,
Null.

IACV or try changing the thermostat.

IACV or try changing the thermostat.

Thank you for your reply.
a question of mine is, why would the IACV or the thermostat make the rpms go up and down. Just curious, I will do as you told me !
Thanks again,

Try resetting your ECU. This same problem happened to me the other day when i installed my short ram intake. My heater was not working for a little while too. The same thing happened, when i would start my car it revved to about 3000 or more and slowly dropped down to about 1800 maybe. Then when it would drop down to about 1300 it would kick up to 2000 and back up and down. I reset my ECU and the problem was solved.

oh damn, i just thought about this. not too long ago the same thing was happening to me. just rev'd like a mofo. it turned out that my engine had vibrated my idle bolt loose. i could not figure out how or why my rpm's were doing that until i went to tighten it. then bam, it worked. see if that could be it. and try reseting the ecu too.