A vacuum leak can cause this type of reaction as well as the TPS (throttle position sensor). Possibly other things as well... dunno.
-Vacuum leak(s):
"Get a can of carburettor cleaner, let 'er idle, and spray anything that's a vacuum junction - including the manifold to head mating surfaces. The idea here is to "fog" the connections - anywhere it gets sucked in is a vacuum leak.
Repair line leaks,"etc.. - 5-90
-TPS testing:
Scroll down to the 4th faq on the page...
http://www.lunghd.com/Tech_Articles/...iagnostics.htm
~Good Luck