1998 Ford F-150 Rough Idle

After a fill up on a trip this summer my F150 (5.4) suddenly began to intermittenly idle rough and the check engine light came on. The idle is rough like a miss, the RPM's drop (500) then suddenly will come back up, sometimes the idle is fast (11-1200). Under load the engine runs fine, with the MPG unaffected. The codes indicated a MAF sensor or O2, my mechanic checked both O2's and they're OK; the MAF looked marginal was replaced with no effect. He is stymied, says he thinks its an intermittent vacuum leak but can't locate one; also the computer doesn't register a misfire when it runs rough. I've had 3 coils replaced in the past, prior to this, losing a coil made it run rough constantly.