Don't chase down the throttle switch, that ain't it.
It also isn't vanagon syndrome, not even close.
Could be a worn clutch/flywheel or oil on it.
Could be a loose motor mount or transmission to engine bolts.

If it is just oil on the clutch, try burning it off by slipping the clutch by starting out in 2nd gear.  But just take quick look to see if the motor mounts look good.