I have a '94 C1500 with a V6 and 4L60E. When I drive over rough sections offroad going fast it jumps out of drive and into neutral. This week I ported the throttle body and added 4 degrees of timing and now it comes out of drive when I stall the converter. This is very annoying. The truck has around 208,000 miles. Any ideas why it does this? Any help is very appreciated.