my 2000 jimmy lurches between 40 and 50 mph. know why?

I'd take a look at the EGR valve for correct operation....might be choked off with carbon....the IAT sensor can cause a rich condition too~ the fuel pressure regulator can starve of fuel as well if is not working correctly....just a few...get your OBD-II diagnostic codes read with a 15 dollar scanner~

could this be a spark related complication...like the ignition wires come too close to the hood 'cause the motor mounts are allowing this change in geometry which shorts out the high tension system~....just another thought~