my friends truck won't start. he doesn't have a computer so I offered to check on line for him. his mechanic changed the starter, negative battery cable even the transmission neutral switch. there is power in the dash & lights but sometimes it won't turn over. he'll get out & rock the truck by hand it starts. any idea?

 

Could you be a little more clear?
When you state it won't turn over, is the starter cranking the engine?
If not, what does it do?
What engine is in the vehicle?
Why do you feel a computer will help with this problem?
Does the vehicle have any anti-theft type system ?