How old is the big chainring? How old is the chain? I'm thinking there's wear on one or both, making it difficult for that last tooth that was on the big ring before you shifted to "give up" the chain and let it onto the smaller ring. If it happens only in one or a few particular crank positions, then I would suspect one or a few problem teeth on the chainring. If it happens "universally" (no matter where the crank is when you shift), then I'd lean toward either general wear on the chainring or the chain. Do you have a chain wear checker?