These are great suggestions. My first thought would be coil and points, 1994 I imagine that is standard ingnition. If it truly is backfiring through the carb you know it is an ignition problem. If you are unable to properly time it (I can provide more detail) your timing chain/belt might have jumped or you have worn lobes on your distributor. With a 1994, the chain/belt could be worn. Since it is intermittent in nature, I doubt it is a chain/belt issue.(obviously thinking as I type)
I would replace coil, points and reset the timing, run it and see if the timing advances if it does you have a chain/belt/lobe issue.