I have a 97 with the 4.2L motor and about 65,000 miles on it.

Last week it started running horribly rough and backfired some. I took the first line of attack and put in Heet. That made a small improvement but still rough. I then changed the fuel filter which brought even more improvement. I then swapped the PCV out and it cured the problem... except between 1,500 and 2,000 RPM. It's not bad but it has a "shudder" right in that area. Above and below runs great.

The check engine light will flash sometimes but doesn't stay on. My local dealer demands appointments and won't run the codes for me. The only diagnostic code readers I can find are $200 so I don't know what the engine thinks is wrong.

Any ideas? This only in a certain range trouble has me worried.