86 s15 Jimmy running terrible

All of a sudden my 86 s15 Jimmy takes a year to start. when i drive down the road, the vehicle slows down, then speeds up and slows back down.
As this is happening, the lights dim when it bogs down and they brighten back up when it speeds up again. Almost runs like its bad gas i guess. Is this a possible sensor problem. I replaced the alternator and no change in lights dimming and getting brighter.:banghead:

All of a sudden my 86 s15 Jimmy takes a year to start. when i drive down the road, the vehicle slows down, then speeds up and slows back down.
As this is happening, the lights dim when it bogs down and they brighten back up when it speeds up again. Almost runs like its bad gas i guess. Is this a possible sensor problem. I replaced the alternator and no change in lights dimming and getting brighter.:banghead:

What engine?
How is the tune up end?
Any check engine lights?
First thing would be fuel filter and fuel pressure?

Thanks for the reply. The motor is a 2.8L v6. The check engine light is on and has been on since I received vehicle from father in law. Town mechanic plugged vehicle to a computer and no codes came back

Thanks for the reply. The motor is a 2.8L v6. The check engine light is on and has been on since I received vehicle from father in law. Town mechanic plugged vehicle to a computer and no codes came back

If the light is on there is a reason.:grinyes:
Ground A and B on diagnostic connector for codes.
Go over tune up stuff.