redneck 4 life

Yea i know its not a chevy but its mostly the same and not mine. So my friend has i thinks its a 96 GMC sonoma, wellsometimes when he gose to start it it wont start he gets nothing like the batt is dead. he waits a few mins and trys again it starts, but it idols rough and the speedometer jumps around from 0 to 60 then from 0 to 100 the engine is just idoling though. they changed the batt, checked the altenator and it was fine now its doing it again. o yea and twice while going down the road it would just quit and then when he started it it did the jumping thing so i dont know what is up. Does anyone have any suggestion to what this is.
Thanks

shawnvw

start with the ground under hood. Make sure it has a clean and tight connection on the body and on the motor