Exhaust riser coming loose

[P69]

I replaced the exhaust riser on my Universal M50 and, after just a few hours  of engine time, it has come loose from the exhaust flange. The flange is good and tight,  it's just the street elbow that screws into the flange. 

Looks like the vibrations from the engine works it loose.

I tightened it real tight and used that orange high-temp permatex stuff to seal the threads. i guess I didn't tighten it enough.

Has anyone run across this problem and been able to fix it?

Thanks