1994 Ford F250 Hard Shifting SOMETIMES

In my 94 F250, sometimes I can start it up and drive it, with no problems. Most of the time though, when I start it, it revs up way too much before shifting, then slams into gear. The next few gears only engage when I slightly let off the gas. Also, sometimes when I start it, it seems to be missing at first, but when I drive it for a few seconds, the motor is running fine. Any clues as to what may be wrong? Thanks a lot.

Bill