My 83 300DT has had what (and maybe is) a failing transmission, but is seems to have a personality- sometimes it works great and really snaps into gear properly and often it simply slides, quite softly, into the next gear. Levels are fine and I have had the filter and fluid replaced within the last 3000miles. No difference

It has been doing this for the last 2 years with no change.. Any tips on making it consistantly snap into gear without replacing it.

(it has 278k miles)