As it sounds, and I'm guessing here, seems the threads in the pipe end are stripped, not the grub screw itself, right? If that is the case, you might try finding a grub screw just a bit bigger than the one you have and screwing that in. It may be enough to cut some fresh threads to grab the post to hold the pipe steady. You can also tap the pipe, if you have a tap and die set.
Last idea I have (and this is if you run out of ty-wraps) is lather the grub screw in blue loctite and let it dry before putting it in. It may be enough to hold your pipe through a race, but given the way I drive (and the luck I've had) it may make it from your pit table to the track, then tank on ya!