When I was testing my max speed, And I switched to gear from 4 to 5 with high rev, suddenly the engine started to scream, like too much rev, and then settled, but even with max open throttle, it was staying on 50 mph, for minutes. No crawling up, constant 50. And slow revs.

When I swicthed back to 4th gear, it was rapidly went up to 60, but the rev sounded too much, so I switched to 5th again: rev to the skies, screaming, then settling down, and went back to 50mph max.... again.

Like the 5th gear was a 3th.... (except slow revs even with max throttle)

Any idea what's the cause?