Truck idling down and wont go over 40
 

Ok i have a 1997 12 valve with some problems. Truck drops idle and voltage at a stop in drive with foot on the brake. Also it recently started to die and will not go over 40 unless im going down a hill. Just replaced alternator, lift pump, and valve cover gaskets. help appreciated. Thanksssss

Sounds like a fuel problem. First check would be fuel filter then clean screen in fuel heater then replace OFV often this is over looked. Then i would check tension on gov. Springs thats where i would start

i wonder if the crank sensor could be acting up. i had a charging issue that left as suddenly as it came before i had a chance to figure it out, and i did some reading and the crank/tach sensor can cause charging issues as the voltage regulator is computer controlled. it could be affecting the transmission too but i dont know that for sure. can you hear the idle dropping or is the tach just reading funny? could be sucking air into the fuel where the lines go into the lift pump. they could have losened off perhaps