odd revving and stalling in idle (97 Tracer)

I've a 97 Mercury Tracer, automatic with about 110K miles. In mornings when I start it up, the morning will be about 80 degrees outside, as soon as I start it and take my foot off the gas, it will
drop to about 500 rpm, and often stall. If it doesn't stall, it will rev itself up to as high as 2500 rpm for a few seconds, then drop back down to a stuttering and puttering 500 rpm. It will keep alternating between these two extremes with the difference becoming less each time, until after about a minute it will rest and stay at a nice 1000 rpm.
Any idea what causes this? And aside from a complete tune-up (which it really does need, and I'd like to have done as soon as I can afford it,) how to keep it from happening?
Thanks! Liam
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