1994 Isuzu Pickup Engine Rev

Vehicle information: 1994 Isuzu Pickup 2.3L 4-cyl carbureted engine 5-speed manual transmission, 2 wheel drive.
Last night I started this truck up to drive home from the airport. The
truck started and drove normally initially. When I exited the highway (after about 10 minutes of driving at about 55-60MPH), I depressed the clutch, and the engine RPM stayed high (don't know exact number since the truck doesn't have a tach, but I would estimate about 2500-3000RPM). I tried placing a load on the engine by turning on the air conditioning (including compressor). That slowed the engine down a little, but not significantly, and the RPMs climbed back up when turning the A/C back off. I revved the engine a little in case the throttle cable might have been kinked or something, but the engine just settled back to the same high RPM when I took my foot completely off the gas. After another five minutes of stop and go traffic, I arrived at the apartment. I parked the truck and when I turned the key off, the engine wound down, spun up to life again briefly, then shut off. I then attempted to restart the engine, and it refused to start. It turned over, so the battery/alternator (which were replaced two years ago) still work fine, as does the starter.
This morning, I went to start the truck, and it fired right up. I drove it back to the airport, and the symptoms never came back...it ran as normal. I even stopped for gas halfway to work, and it started right up after getting gas as well. What could be the problem?
Environmental information:
The truck is probably not driven more than 1,000 miles/year. It spends most of its time in the airport parking lot (I am a pilot, and the truck is where I am based...I actually reside elsewhere...it is only secondary transportation). The night of the symptoms, it had been sitting for about three days since its prior drive. It was also the first real cold snap of the season during the night of the problem. The car is very dirty (including the engine compartment), from the dust that is extremely prevalent in the Airport parking lot (Newark, NJ). I keep a clean air filter in the engine at all times, but I don't clean the engine compartment itself due to lack of resources/time.
Please help diagnose the problem...I can probably fix it, and I have an idea of what's wrong, but I want to get other folks' opinions first before turning wrenches.
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