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The shifter mounts with a series of bolts around the flange. They could be slightly loose allowing the shifter to slip sideways with extreem use. I would open the boot etc. and check the alignment / tightness of the bolts. that should fix your problem.
Mine moved enough that it would not stay in 2nd, 4th or 6th. This was with a Hurst shifter and powershifting.
On the stock shifter there is a neutral pin on the front of the base. It sits in a hole there waiting to be used. You use it by putting the shifter in neutral and inserting the pin in another hole which should line up with a hole in the shift shaft. This shaft is the pieced which has the hole in it for the shifter and is not removed or changed when installing an aftermarket shifter. On the rear of this shaft is an adjustment where it is clamped to the rod that goes back to the transmission. If the neatral pin does not line up properly, loosen the clamp, insert the neutral pin and tighten the clamp good and tight.
This adjustment cured the popping out of gear problem, but not the trashed syncro's.
There also could be worn pads on the shift forks and/or worn shift forks. The steel shift fork goes a long way to cure this problem.
Check out this link for pictures of my transmission rebuild.