904 won't go into gear. Sometimes. :(

It may be drainback, but if it takes longer for it to work when it's cold then it's time for another rebuild.
How long does it usually take to engage when it does it?

Drainback is caused by a pump and converter bushing wear.
It takes a couple of seconds to engage when you put it in gear after sitting.
Transgo and a few other make a manual (shifter) valve that lets the trans fluid circulate in park that will help a lot with drainback.

Try poping it into neutral for a couple of seconds before you try to drive it, and if it works fine when you do that it is drainback.

If it's cold and you put it in neutral for a few seconds then try to drive it and it won't go till it warms up for a few min then it's hardened rubber seals and needs a rebuild.