having some transmission problems

i am having some problems with my c-4 trans,
when i first start it goes in to gear great and shifts great, but after about 10 minutes of driving, reverse still engages correctly but going into drive has about a 2-3 second delay before it engages, and it still shifts firm
after a couple more minutes of driving, reverse is still ok, but it wont engage into drive at all unless i rev it up to about 1500 rpm, and then itll drop into gear but when it goes back to idle itll drop out of gear again, and the shifts feel sloppy, but once it gets into a gear it doesnt seem to be slipping
fluid looks ok, but it was a little overfull so i corrected the fluid level and its still acting up
heres the info on it
1968 c-4 rebuilt a month ago by me with TCI rebuild kit
TCI shift improver kit
Hughes 2400 stall convertor

what could possibly be causing this,
maybe its something i messed up :nonono:
i dunno, any advice would be great
thanks in advance