If that doesnt work, un-do the rocker arm bolts and remove the rocker assembly. Take out the pushrods (remember the order you remove them with!!) and roll them on a flat surface and see if any are warped. If they are, get a new set. IIRC it's not too expensive.
If not, then maybe you have a sticking lifter (a problem I had). If this is the case, you'll have to remove the head and obtain a telescoping magnet to yank the lifters. Fortunately it sounds like your engine is probably run regularly, so the lifters should be easy to pull (in cases where they're stuck, sometimes you need to pull the oil pan and drive out the lifters from below).
