[Bill Baer]

Ok Bob here is another possibility, if for some reason your trans is not completely in low ,like a mismatched shifter/shift lever in the trans or the shift cable is not mounted to the trans you may not be getting the trans in completely in low and when the motor torques up it may move just enough to put you in high gear and of course the trans brake doesn't work in high grear. Watch to see if your motor moves back and forth when you load the brake it doesn't take very much movement to cause this to happen also make sure the shift lever is correct I can tell you from experience the the Hurst quarter stick is different from most of them.
Regards:
Billy Baer

P.S. Please don't ask me how many times I removed disassembled, re-assembled and re-installed my transmission(s) until I finally figured this particular gremlin out.