JD 214 slowing/stopping up hill

First post but thank you for all the information I've received from this site.

I have a solid JD 214 that I'm recently having issues with it slowing/stopping up long gradual sloping hills in 3rd gear when the variator lever is left on slow (towards the back of the tractor). If I move the variator one slot forward the tractor lurches forward at the normal speed for this variator location (too fast to mow). I replaced both drive belts and adjusted the variator per the operators manual. I even pulled off the mower unit and the tractor still does the same thing. I jumped off the tractor and could tell that the primary belt is slipping (the belt between the mower and variator). The engine is running strong through the entire process. The variator spring is only half-way through it's possible adjustment-could this solve if I tighten? Do variator springs need to be replaced?

Thanks in advance