Is your initial drive set to the lowest setting?
The slipper is a tuning tool, I adjust for available traction. The buggy will still run like a bat outa hell with a looser slipper setting it just softens the initial torque allowed to hit the drivetrain. It is especially beneficial when trying to eliminate the above bolded sentence.

If you can run a tight slipper with the traction available at your track, great. If you are doing doughnuts while everyone else is driving towards the next corner, losing a little straight line acceleration with a slightly looser slipper but being able to stay with the train to the next corner will net you a better finishing position.

Many suggestions made, try them one at a time till you can get your buggy locked in and competitive. Stiffer spring in the front as suggested a couple times is a logical step along with the other adjustments, which one or combo of them gets you locked in? Test & tune to find out