Drove the car for an extended period for the first time and encountered a problem. Once it was fully warm and had been driven for a while (~20 minutes), the motor surged at idle, pinged and stumbled badly when anything other than a very slight throttle was applied. If it were only pinging, I would attribute it to crappy gas, drain the tank and re-fill it, but the idle surging has me concerned that it might be a deeper ignition or fuel injection issue...any ideas?

Could be a vacuum leak. I've seen several of the rubber hoses, AND the plastic lines break while driving. Might do a good check-over on all of those first.
Bad gas can also effect the O2 sensors, and cause a rough/surging idle, and pinging.
Do you have access to a scanner with live data?

I also experienced the same stumbling problem with my '91.  The first things I checked were the easiest; spark plugs, vacuum lines, fuel filter, fuel lines, etc.  I thought it might have been bad gas but it wasn't.  What I found was a bad ignition coil.  That's not to say your problem is the same.  Check the easier stuff first.  Good luck, let us know what you find.