1997 Hyundai Accent intermittent starting difficulty

Engine Mechanical problem
1997 Hyundai Accent 4 cyl Two Wheel Drive Manual

had coil, wires, & plugs replaced 1month ago.
now car wont start on occassion. Drive to work fine go out at lunch car cranks but will not start.
if I leave it overnight & come back in morning it will start right up. Or even an hour later. What could it be? A friend tested it for spark there was none. But again later it will start. At shop 5.8.08 they looked computer does not show any problems. They are baffled! Any suggestions, it is so frustrating to get out of work and have car not start