Sir--first off, I've never run a 914P, I've had similar experiences on auger head crossauger slipclutches. What happens is the slip clutch assembly gets "polished" from slipping too much, I have a couple that are literally polished to a mirror finish. Once that happens, they won't hold no matter how tight you make the spring. You can hold them in your hands and press them together and turn, and they just spin-even though the "waves" are not worn down, just polished. OR--you have the slip clutch way overgreased, with grease in the actual clutch ring, not just the shaft. Same principle.
Get new clutch "wavy rings". They are not very expensive. Install and tighten to spec, and I think you will be surprised how much better it works.