Ed,
If it's not happening all the time there's a pretty good chance it's the coil packs. When you get the coils out, have a good look at the clearish plastic window on the side and see if there's any burn marks, bubbles in the plastic, cracks etc, etc. Also, pull the rubber boot off the end and check out the carbon contact ( and spring ) to see if it's shot. The carbon bit contacts the end of the spark plug and can errode over a long period of time.
When I had my HR31, sometimes the car would be OK but when the coils packs got hot, the fault would show up. I replaced my set with a set that were in much better condition and problem solved.
If you do need coil packs, give me a yell. I've got a bunch of really good condition ones.
Otherwise, have a good look at spark plugs, wiring, connection plugs etc for general condition.
Hope that helps you some