I havent had a ton of experience in this area (probably a good thing) but that sounds like you may need some work on your clutch packs. If the fluid levels are alright and the fluid itself is good (didnt say when the last time you changed it was) I would start looking at the clutches or possibly a broken shift valve. I think there is a known problem with these on the 86 and 87 300E's...I would be interested to see what some others say on this
