Chevrolet Repair: Random Multiple Misfires - 97 Chevy Astro, astro awd, power steering pump

Question
Hi - we've about reached the end of our ropes.  (I'm the wife typing for hubby - so hopefully this will make sense) We've recently had to do alot to our 97 - just replaced tranny - power steering pump, serp belt. Also, when we got it back from the tranny shop, the fuel pump that had been threatening finally died (so now my gas gauge works, too!.)  After everything we did, we had what I can best describe as a "slamming" miss. Scanbox said left bank, right bank rich. He got that to go away.  #1 fuel injector was bad he was told by the guy at the tranny shop - replaced that. Still missed. Replaced #1 plug and wire because the boot was cracked.  This took away the #1 misfire.  The van still is missing periodically.  We test drove it last nite -- 50+ miles.  For varying lengths of time, it would go along fine, then - slam or huge hiccup.  Then it would straighten out again.  We plugged in the scanbox again.  Now the code reads Random Multiple Misfires.  What are we missing?  What are we overlooking?  We are scheduled to have the other plugs and wires changes on Monday at our Mechanic - (last resort) we tried to get to the others, but unless you stand on your head under the thing, you can't get to the plugs without a rack.

Also - gas mileage basically doesn't exist!  We're down in the low to mid teens now.  I was getting 20+ before the tranny went out.  

Any ideas?  My hubby was trained years ago as a diesel mechanic, and has worked on other gas engines, even rebuilding, so he at least knows his way around under a hood, but this one has him stumped and frustrated!  Vehicle is a 1997 Astro AWD with 150,000+ miles.  Most of the driving is open road - very limited city.

Any help you can offer would be great appreciated.  THANK YOU!

Answer
Hello L&J,
That plug and wire tuneup is a good start.
If they are that hard to change, they have probably not been changed in quite a while.
That slam you feel is probably from raw gas getting into the catalytic converter, and building up, then exploding.
Sooner or later it will rip the muffler open, or melt down the cat bed.
May have already done that, and the cat could be restricting the exhaust flow.
I would,(after the tuneup) disconnect the exhaust in front of the cat, and go for a ride. See if it runs better that way.
There are ways of having the restriction measured by drilling a hole and using a gauge, but I have never done it.
But unburned fuel in the cat will melt everything in it to a lava like mess, and plug it up.

That could actually cause some misfires, but long term misfires is what causes it to melt down in the first place.


Van