Transmission went. I got a used one, then that broke for the same reason.
If you own one of these, read this, it's very important!
Universal problem with 93 and 94 manual transmissions. Easy fix around $100.00. The nut that holds on the 5th gear assembly is improperly staked from the factory. It will come undone. Then your transmission will have to be rebuilt... very expensive. To fix this, you do not have to remove the transmission. There is a plate next to the drive shaft. Remove that. There is a threaded shaft with 2 groves on each side. The gears are pressed on the shaft. There is a nut that holds the gear (reverse and 5th 1 gear). The nut is torqued and then staked or crimped into the groves. The idiots at Mitsubishi didn't do it right.
Do not reuse the nut, buy a new one for around $10.00. It is not a matter of if it comes apart, it is when.
Other than the transmission hell, this was a great handling car, but you have to align it really tight or you will wear the tires quick.
The valves do make a lot of noise also.
Same thing happened to me. While driving down the road I heard clunk. Turns out it was that nut you were talking about. It wore a hole in the cover and shot out. A few miles later my car would not hold fifth gear. The only rebuilt transmissions I could file had 1 million miles on them. I ended up junking the piece of junk.