I was thinking +1 on the slipper clutch right up until you said it will slow to a stop with engine at full RPM. That is definately NOT the slipper clutch prividing your description is accurate. I'm sitting here scratching my head trying give you a start point and am a little short on information.
IF, it hooks up and launches as in, motor is audibly loaded and increases speed as rig increases speed then just stops pulling and stops with motor still reving, look at main clutch, disassemble and check for signs of excessive heat.
IF, the motor pegs straight to top rev and stays there during rig acceleration, then you DO likely have a slipper clutch issue. Easy check, leave the body off, nail the throttle out a ways and nail it to bring it back to you. when it returns, reach down and put a finger on slipper clutch. If it burns the crap out of your finger, it's way too loose.
But that still doesn't explain the slowing to a halt with motor reving........
UNLESSSS your description of what it is doing is more a description of what it did ONCE. If that be the case, your slipper was way loose, you smoked and burnt off the pads and lost all contact with your slipper clutch.
check out the slipper clutch first, make sure you can barely turn spur gear while holding shaft still. If it turns free, there's your problem.
Re post when you at least got those two things looked over good.