My first thought is to make sure your TV cable is tight at wide open throttle. It could be a clogged cat like Jim said, but that wouldn't be exclusive to just 1st and 2nd gear.
My thoughts are possibly low fuel pressure regulator, clogged fuel filter, or a fuel pump that can't supply the flow rate required at higher RPM's.
Also check your battery voltage and your charging system. A low battery won't spark at higher RPM's. If your alternator belt is loose, that could cause it.
Also it could be a vacuum leak at the manifold top, throttle bodies, or vacuum lines, which would throw off the MAP sensor and cause the shift points to be missed.