[nutsandbolts]

my 89 voyager starts up fine then after a about half a minute of running the RPM drops down to near stalling then RPM goes right back up, sometimes it does stall out and restarts okay. This goes on for 30-60 minutes of driving, maybe longer. It seems to be worse during rainy days, but not sure. When I floor the gas peddle under load, it sputters like crazy. My cruise control is not working and not sure if it is connected to this problem. I'm getting only 13 mi/gal too. I did a code error check using 'check engine' light method and got 13, 54, 55. I believe 54 and 55 are both 'completion of fault code display'. 13 is MAP sensor, but if something else is causing the drop in RPM then manifold pressure would drop causing a code 13. A vacuum gauge shows strong and steady vacuum except for 'RPM drops'. I did new gas filter, plugs, wiring, and rotor, with no change to stalling problem. Any help appreciated.