Chain pops on cassette

I'm trying to help a buddy on this one. He has an older 29er hardtail. He’s a big guy (250ish) and puts a bunch of road miles on every year. He has a new chain, newer cassette, & rear derail looks good. When he goes uphill the chain pops around a little on the cassette. When not under load it’s smooth & shifts beautifully.

Any thoughts? I’m out of ideas here. Thanks crew!