problem transmission skylark 82

Hello everyone
I have a skylark82 equiper of the engine v6 2.8 and transmission th125c.
I have a problem with the transmission, I can go back and back without problem but impossible to advance despite the fact that the speed passes correctly.
when a forward gear is put on we hear a snapping noise in the transmission
I am lost and do not know what it could come the transmission drain was made the oil in the sump was clean red color without deposit
do you have tracks for repairs?