Probably a worthless idea but in my jeep when I put a load on it and the drivetrain torqued it would pop out of gear then when I let off it would usually pop itself back in. After some head scratching I discovered that the shift arm was hitting the hole in the body causing it so push it forward a little. So, maybe somethings catching your shift linkage while under heavy load. I don't know much about tranny's, car or human form, but as far as I know synchros are just for matching the gears between shifts. Most semi truck tranny's don't have them and a lot of old European car's don't have them and they don't slip under load... Sorry for the long post but liquor helps me not shut up
-John
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