to me, it still sounds like worn bushings. you said yourself, it all worked good at the start of the season.
with the car on the ground, in first gear. lay on the ground near the rear wheel, and rock the car back and forth, look to see how much movement you're seeing with axle roll. you might be getting the wheel hop from the bushings loading up, and letting go as you lose, and gain traction. Like i said earlier, i went with heim joints in my toque rods to get rid of any of that wrap and i the axle doesn't load up at all anymore. i think between bad torque rod bushings or bad main axle bushings thats where you're getting your hop.
im not gonna dwell on whats proper and whats not, but a pan hard bar simply positions your axle laterally.. so i can't see it affecting vertical performance.
Not sure what it is worth, but this is my vote. you may need to get more aggressive with trying to rock the car, use your e brake as well as first.
dead bushings are guaranteed undampened play, don't waste your time on anything else until you know they are good.
I would bet if the panhard bushing is bad, then the lower axle bushings are toast (unless they were already replaced)
If you have not done them, now is a good time to do them.
Stiffer springs and shocks will exacerbate any issue with the bushings.
The following advice has already been given, but i will reiterate it.
When i first got into these cars, i put full poly in the back, which is a mistake unless you just want to drift and do burn outs, made my car worthless in snow and rain.
Stay with QUALITY rubber, or go with heim joints.
To me, poly was ok on the the panhard rod though, and seemed OK on the front trailing arm bushing, and front torque rod bushings, but 100% does not belong in the trailing arm/lower axles bushing imo.
+1 on tire pressure as well.
i have been a bit out of the game for a while but i think kaplan has heims for the lower axle to trailing arm bushing, someone else may too.