It sounds like a problem with the inching valve or the inching master cylinder. If you take of the floor board and depress the left hand pedal you should see a lever move on the top of transmission. That is the inching valve. Either it is working intermittantly or not returning properly. There is a spring and seal kit available for the inching valve. It sounds like an easy fix (for someone that is familiar with forklift trucks). You might want to call a dealer and have them repair it before you go ripping things apart.