Going uphill Miss fire

I have 2003 GMC Safari, passanger van, 4.3L, 180,000km. Twice we were going up the same large hill, engine warm (highway driving for an hour), van loaded with family of 7, check engine light starts flashing...miss fire. Engine light stays flashing till you stop, shut off engine. On re-start, no more flashing. I'm not planning on fixing this myself, I just would like to hear some ideas before I go to the mechanic.

1) How old are the plugs?
2) How old are the plug wires?
3) How old is the Air/fuel filter?

Can you get the Diagnostic Trouble code(DTC) pulled for free at any local
Auto parts store for free.

Need to find out if your are getting random misfires on random cylinders, or single misfires on a single cylinder.

Can be a lot of things which have to be ruled out. I've had similar problems on various vehicles. On my GMC Jimmy it was a clogged fuel filter. Then on another it was a bad O2 sensor which caused EGR to carbon up. A weak spark can cause those symptoms as well.

when was the last time you had a tune up? I would check the cap, rotor and wires and then check your timing. You also want to make sure you have ac plugs, everyone says to use ac plugs and I personally tried other brands on my 02 and the van didn't like them. Switched the plugs back to ac and problems went away.