Originally Posted by sqzbox
Check ground strap from engine to firewall also.
If the engine is not grounded to the chassis and body then it could be loosing ground. You should have a ground from the body to the frame, engine to body, engine to frame and battery ground from battery to body/frame and it wouldn't hurt to have one going to the engine. A lot of times these ground straps get removed.
Had a fellow that had an older ford truck. When he turned on the headlights it ran bad, when he turned them off it ran good. Turned out that the ground strap from the body to frame was missing. When the headlights were on they were grounding through the ignition module that was mounted on the fender well.LOL
As you turn the engine could be shifting and loosing its ground.