Look at your HST linkages. Follow the rod from the pedal link back to the transmission. Right there at the transmission's trunion arm, there is a ball joint, the rod is threaded into it on the front end, and the other end that goes through the trunion arm has a nut on it. 5/16-24 as I recall. Well they didn't put lock nuts on them. Just a nut and lock washer, and they sometimes come loose/fall off.
.....this is assuming that you're talking about the HST being slow, yet engine runs perfect.