Engine Performance problem
1988 Ford F150 V8 Two Wheel Drive Manual 56,000 miles

When I start the truck the idle holds at 1500 rmp's and than after about 60 seconds falls back down to around 700. After I begin driving, the motor will surge to 2300-2500 rpm's and only come down when the clutch is ingauged. I changed the IAC motor thats under the throttle body, and it seemed fine but than started acting up again. Where do I go from here?

are you sure you replaced the IAC module or was it the TPS??

was the old IAC module all gummed up?