If it is a manual trans. the only thing that could slip like that is the clutch. That generally doesn't happen that way from sitting. Its usually the other way around. the disc sticks to the flywheel & wont relaese. If you have a removeable lower pan under the flywheel You can look up in there & see what is happening. How about a clutch adjustment. There has to be at least a small amount of free play in the pedal. About1/2 to 1in. is normal.
