Bike feels like it's slipping/shifting gear when pedalling while standing up on max gear
Chain is probably jumping on cassette.

This could be just a stiff chain.  Clean and lube the chain.     

If not a stiff chain then more likely a worn chain and or cassette/freewheel. When they are worn they don't match up well and can jump. 

If by lower you mean mean the bigger diameter gears they are less likely to jump as they have more teeth touching.  

You can get a relative cheap tool to check for chain stretch.  If the chain is stretched then replace it.  Cassette/freewheel is a little harder to measure wear - I bring up a picture of a new one and compare.  Or just buy a new one and compare.  Or just post a picture of it in you question.  If you replace the cassette/freewheel then always replace the chain.  You can get like 2-3 chains per cassette.