Going down the highway or road ( 40-65 MPH) and having the car at about 1500 RPM's my car starts jumping up and down on the RPMs like crazy about +- 300.

What would cause this?

Other that that the car runs great at WOT. and other RPM Ranges.

I know about 2 months ago we put a used set of heads on it. and my dad kinda rushed adjusting the valves. But if it was a bad adjustment on the valves wouldn't the car act up more than just at 1500 RPMs?

Thanks