i'm with Justin on this if it happens as you start to move believing you're in gear. The shifter plate would be my first stop.
At first I read that you were getting grinding as you put the car in gear and if that's the case I'd make sure I'm pushing the pedal fully down and nothing is bunched up under it, then look to the clutch cable adjustment.
Failing those a loose trans mount or it's internal. Synchros or shift arm.