could be two things:
1 the ones you put in were not in good shape(even thought they were suppose to be OEM, were they rebuilt??if so they might have messed up???
No.2 you might have a bushing(s) someplace in line that is worn and letting them vibrate and this will let them go out in a hurry???
in the late 60's i had a 66 mustang 3 speed standard, 6 cyl. that would blow one transmssion after another.. back then you could get an old transmission out of a junk, mustang, comet, falcon, etc, for about $50... so it was no big thing. i would just put a new trans. in and drive on...
well anyway in 1973 when i pulled the engine out to have it rebuilt i could see the pilot bearing in the crankshaft was pretty worn... and this is where the front shaft on the transmission was inserted to hold the front shaft.. it took me a long time to find the problem, but after the bushing was replaced there was no more problems with transmissions in this car....
