My 1989 honda accords transmission shifts just fine and really smooth when it is cold but when it gets warm then it starts to slip between 2nd and 3rd gear like it will not go into 3rd gear unless you have it in d3 and take it to 4000 rpms and then shift to d4 and sometimes it will go into 3rd like 2 out 5 times it will go into 3rd but other than that it shifts very smooth

When is the last time you serviced the transmission?

If changing the transmission fluid does not solve the problem, one or more clutch packs are likely shot.