Engine won't shut off

I have a 1970 Chevelle (rebuilt 327 2 bbl) that starts and runs great but won't shut off. The problem started happening when the car was in the shop for a new trans (a thm 350 to thm 400 swap), currently the kickdown is not connected. The mechanics thought it was the ignition switch so when I got the car back, I replaced the switch which worked the first 2 times, didn't shut off on the third time and then shut off the engine 3 more times(worked for a day) and now doesn't work. I would be suprised if the switch is bad at this point. It's a brand new Delco switch I got from a Chevelle parts house. I have doubley checked the position of the switch (installed in lock position) and the switch is tight on the column. It switches lock to acc to start smoothly, nothing is cathching. Also I can see the linkage in the column shifting the switch smoothly at the switch. My second thought was that maybe the TCS system (since it is connected to the ignition was the culprit, but that has been disconnected electrically and the vacumm is going directly to the distributer. The distributer is an Accel super coil HEI. What am I missing? Thanks, Thom