It's almost certainly a throttle linkage lubrication problem - this is commonly first noticed in the late fall when temps fall and the old lubricant hardens up due to the lower temps. After a few minutes of operation the engine compartment warms up and things are back to normal.

The correct lubricant for throttle linkage pivots and balls is standard ATF. Unfortunately a couple of the lube points are very tough to get at since they are buried under the intake manifold. Lube up the ones you can get to and see if that does the trick.