30mph bouncing off a rev limiter??

Hey all,
I recently bought my son a 2011 800S with 3100 miles on it. We swapped all the fluids out and put new harnesses in it. Cleaned out the clutch and air filter with compressed air. The PO put new plugs and wires on the car. I took it out for a spin today after getting it all back together and it sped up to 30mph quick and easy. Once it hit 30, it was bouncing off some limiter and wouldn't speed up. There is no seat belt wiring that I can see (pans are off the bottom so I can see too) and all "Seems" well. I took it back out to check the rpm's and see if it's clutch as suggested elsewhere. When I took off, it went up just fine and kept climbing. I think I hit 50 before I let off. Strange. Is this intermittent case sounding familiar or does it indicate anything to anyone? My son is moving from the 170 up to this and he'll not be too happy to have the same top speed in a bigger machine.

Anyone?

Thanks
Dave