1998 GMC Nightmare

I have high mileage 98 Gmc 1500 sle transmission is csrewy, transmission has no codes, and transmission mechanic says he thinks tranny is OK, just recieving a code from a bad sensor, no codes show at all? also there is a idle problem, the same idle problem occured on previous motor, doesnt do it all the time just enough to wear down my breaks by not idling down at stops, and wiil run at 40 mph all by itself when I have taken my foot off of Gas, and ocasionally when I put it in park it revs up momentarily, once again shows no codes and nobody has any clues. I replaced fuel filter, I do know that I have some problem with the catalytic converter, but still I have no codes and all these problems,
RU A Genius, I hope so, last person suggested a check at a dealership,
I am leary.

I have high mileage 98 Gmc 1500 sle transmission is csrewy, transmission has no codes, and transmission mechanic says he thinks tranny is OK, just recieving a code from a bad sensor, no codes show at all? also there is a idle problem, the same idle problem occured on previous motor, doesnt do it all the time just enough to wear down my breaks by not idling down at stops, and wiil run at 40 mph all by itself when I have taken my foot off of Gas, and ocasionally when I put it in park it revs up momentarily, once again shows no codes and nobody has any clues. I replaced fuel filter, I do know that I have some problem with the catalytic converter, but still I have no codes and all these problems,
RU A Genius, I hope so, last person suggested a check at a dealership,
I am leary.

I WOULD CLEAN THE THROTTLE BODY OUT WITH SEAFOAM DEEP CREEP. AND CHECK FOR VACCUM LEAKS.ALSO I WOULD CLEAN OUT THE EGR VALVE ALSO. I HAD THE SAME PROBLEM. SO I LIVE BY CHEMICALS NOW.