I have a 1995 C1500 4x2 5.7 liter. The engine stalls when in drive, unless I keep the rpm's up, so I've been shifting manually to keep the rpm's up and it seems to work fine. It doesn't stall in park or neutral, so I've been shifting into neutral at stop lights. Sometimes, even when I keep the rpm's up, the engine loses power, sputters, and seems like it is going to stall, but then regains power all of the sudden. These problems started occurring the other day, and then today, the truck started working fine again. What could this problem be, and why is it an on/off problem?