Before crashing my bike, I had an issue with the chain and the big chainring. Every revolution, in a 53/13ish gear, the chain would ride up the chainring on the verge of falling out. I decided not to use the big chainring for most purposes.
Once I crashed, I ended up getting a new chainring... that fixed the skipping issue. A new chain quickly followed for good measure.
Perhaps the same issue for you... except with a cassette. Not saying you need a new cassette, but not saying you don't either. That's a matter of preferred strategy.
Do you have a record of your most recent drivetrain changes/mods? Perhaps a certain component has run its course.