1999 F100 dies suddenly.

Have #3000 hrs. and started/idled fine. After coming up to speed (#4700 rpm?) and running for a mile it suddenly died. Still cranks but wouldn't fire even with fast idle lever stroked. Fuel bulb was hard. Not over-heating. After kicking in on 9.9HT it firied up 1/2 hr. later and ran fine. Tried running again and couple miles later it died again. Back to the kicker going to the ramp and sure enough 1/2hr. later it firied up and ran fine all the ways to ramp. I'm guessing the CDI is going out but mechanic told me they usually die completely? Previously fried voltage rectifer/regulator and definely not that and fuses ok. I've had trouble with harness before but since splicing been ok and don't think it's the problem. Just had in shop for annual and mechanic replaced timing-belt and tried to reshim valves but 1 exhaust valve has smallest possible shim already.He plugged into harness but connector is tight. Any suggestions? Thanks in advance. Bob