No Power! Ready To Push Off Cliff

I've researched several threads with similar problems but none with conclusions. 98' Duratec, 92,000 has absolutely no pick up when starting off from a dead stop. Had 100k major tuneup (new plugs, wires, tb and inj. cleaned) done at 88,000 with rebuilt trans. as part of recall. Ever since, no power. This is very apparent during hot weather, AC on, stop and go traffic. Moderately accelerate from stop and there is this hesitation followed by a surging as car trys to gain speed and then at about 40 mph car will take off with plenty of power. If you accelerate hard there is no hesitation. No other problems with this car. This vehicle is well maintained (oil, filters, inj. cleaner, etc). No codes. Pulled front three plugs out and rechecked gap and all within specs. However, the first plug did have an oily substance around the top of the threads. Plugs were sooted black but not fouled. I'm at a complete loss. Have visually inspected everything under hood (vac. hoses, wires, etc.) and can't find anything wrong. Dealership here is not the best. Any ideas? Can't imagine going much longer like this.