2000 SS transmission

I've recently purchased a 2000 SS with 34,000 miles. The car is great but I notice RPMs spike 300-500rpm just before the car shifts from 3rd into 4th gear. The dealerhsip has checked it all out and find nothing. They have even gone as far as pulling a new model off the lot to compare it to and they find nothing. I've never seen this in any other car I've driven, has anyone else seen the same thing? Any ideas what it might be?

Pressure