1995 Escort Idles Rough After Warmup

I have a 1995 Escort with around 77,500 miles on a 1.9L engine, automatic with A/C. It will run smooth until it warms up then the idle
speed drops to around 500 RPM in gear and begins to idle slightly rough. It doesn't stall and seems to have plenty of power when accelerating. It will not start on the first try when it is warmed up but will start on the first try when it is cold. The tail pipe looks to be very black but I haven't noticed any black smoke during acceleration or idle. I would appreciate any suggestions on where to begin looking.     Thanks in advance for any help offered,                     Jim
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