I had this problem. Turned out to be the Idle Air Control valve. I removed mine and ran the tests listed in the Haynes manual. The unit tested within spec, so I cleaned out the carbon with brake parts cleaner and a pipe cleaner, reinstalled and it ran fine. Regarding the tests in Haynes' manual, the IAC ohms were 11.0, the manual said the range would be between 6 and 13 ohms.