1997 Hyundai Excel Engine drop out

For a few months, yes I know that I should have dealt with the problem sooner, my car hesitates for about half a second, and then is fine. This can happen often during a trip, or not at all, and has recently been getting worse. The car has recently been serviced, new filters, ignition leads, fluids all done, a general safety check, and the timing belt, some driving belts and the crank shaft pulley done. There appears to be no mechanical problem at all, so I am assuming it is an electrical engine management problem. My mechanic is a bit stumped, as everything checks out on the testing equipment as OK, but the problem is still their and seems to be worse, probably because the engine is working better. Can you suggest where to look or what to start testing so that my car can be fixed?