Jason,
I'm guessing that this hopping is when you are on power coming out of the corner? In my experience, rear end hopping is caused by one of two main problems - too much aggressive power too soon, or too much rear end grip / not enough slip somewhere in the system.
What gearing, motor and cells are you running? If you can (reasonably) increase your gearing this will reduce your punch out of the corners slightly and can help reduce hopping too. Aside from that, feed the power in more smoothly or add some throttle exponential in your handset profiles (if you can do this).
Are you running a diff? If not, add one, assuming your club rules allow them? If you already have one, does it run smoothly (no notchy/gritty feel to it?) or is it too tight? The only real way to test a diff's tightness is to give your car the beans in a straight line from a standing start. If the diff slips it is too loose, so tighten it. If there is no slip is might be too tight, so loosen it until it slips and then do it back up a fraction. It is a careful balance but it makes a big difference! Do not try the usual trick of holding one wheel and spur in one hand and trying to twist the other wheel - this applies far more torque than our motors can usually give, or it will strip the plastic wheel spacer if you are using one! Also, a clean, well built diff always works better than a manky old one... Rebuild time?
As for grip, the 35 rears are great tires, I use them myself in extremely high grip conditions so I can't say there is anything wrong with them! It might be that you need to either come down on the rear springs (kit soft for example) or go up on your damping (I use either 20,000 or 30,000wt in my tube with the medium springs). NOTE - there is a difference between "wt" and "CST" units, I use 30,000wt oil, not 30,000cst oil! Also, what tire diameters and ride height are you using - I never run my car higher than 3.5mm ride height and smaller diameter tires (48mm or less) tend to give less problems with both hopping and grip roll due to less flex in their sidewalls. Last thought on tires, have you rounded off the outer edges? Again, rounded edges help limit the amount the tire can "dig in" through the corner and reduces both hop and grip roll.
Final thing, why 2 o-rings on the rear? I am assuming you have these on the rear guide pin? I must admit that this is the first time I have ever heard of this idea! I will say though that the red VRX o-rings will give a smoother rear end action then the kit black ones. Just lightly lube them with bearing oil every couple of runs and I find they work a treat.
Hope this helps,
James
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