Dies in Drive and Reverse

Okay Guys and Gals I need some help. I have a 70 Dart Swinger originally a slant 6 car now a warmed over 360/727 combo. My current problem is when its warmed up (15 mins at idle for example) and I go to put it in drive or reverse it dies instantly. I can start it right back up with no problems and it will sit idle just fine. Here is what I have done so far different battery, alternator, new voltage regulator, ballast resistor and ECU box. Non of these changes have made a difference. If its of any help the wiring harness is factory original. Any help with this problem would be appreciated.

Ken