Engine looses power and makes a snapping sound on the drivers side of the engine when the engine is pulling uphill in overdrive and when I take it out of overdrive it stops. Now it's does it a little when it's not in overdrive. It sounds like it's coming from the area of the drivers side valve cover. When the engine is in high RPM'S it's fine no snapping or popping and power o.k. . Fuel pump pressure is fine, changed plugs, plug wires, D cap. Maybe loose rocker arm ??? Vacuum problems ??? Push rod ???

 

I was going to go with a plug wire jumping spark to ground but you beat me to it by replacing the plug wires.

 

Broken valve spring on that side?

 

You know, this may sound really stupid, but have you changed your crank sensor recently? If you haven't, you might give it a try. They are cheep, and seem to be the root of many problems. I was having shift point issues and some pinging issues (which can make kind of a popping sound when pulling a trailer, high torque, "low" rpm situations) and all have gone away since I changed my crank sensor.