1999 Chevy Lumina Misses at 1000-2500 RPM occasionally

My 1999 Chevy Lumina misses at 1000-2500 RPM occasionally. It all started about a year ago. I had the fuel injection cleaned and flushed the radiator, changed the computer and the MAF sensor, and recently did a tune up where I changed the fuel filter, air filter, plugs, and plug wires. We did the tune-up in January and the problem went away until today (19 April). Can anyone tell me what might be the problem?

Check the resistance variation on the TPS with a multimeter, it should be even as you slowly move the throttle from closed to wide open. Just one possibility.