1996 S320

[ken1]

I have had my 140 for about 2 months and dearly love it. That being said, I need help. I have noticed on occasion it would not shift into 5th gear. I would move the shifter to the right and it would then shift into high. This has happened maybe 10-12 times over the last 2 months. Most of the time it shifts normally. I have put about 5,000 miles on it since owning it and it didn't seem like a big problem. I thought possibly a linkage adjustment or shifter bushings.I'm taking it to my indy next week for a trans service plus a couple other small things. Today about 50 miles from home, cruising at 75 on the interstate, I heard a "thump" kind of sound from under the car and it went from 5th to 4th and stayed there. I continued in 4th about 4-5 miles until I could get off. It seemed fine except 3400 rpm instead of the 2500 rpm I normally see at 75. I checked the fluid and looked underneath. Everything looked ok. I drove it home. It makes a slight "thump" sound when I take off and when I let off the gas and has it no 5th gear. I tried to research on the forum but couldn't find anything on this problem. Also, I'm not sure what transmission I have. The owners manual says it's "sealed for life" but mine has a dipstick. I researched the 140 before I bought it and the only transmission symptom I found was "clicking in reverse" which mine didn't and still doesn't do.
I'm sorry this is so long winded and I sure do appreciate all the help,
Ken