The other common cause of this problem would be a mis-match of the chain and cassette, eg, a new chain used on an old worn cassette, or vice versa. If the jumping happens only under high-load pedaling, I'd guess it was due to this condition.
For example, I've found I need to replace chains about every 4-6 K miles based on 0.75% wear limits. The used cassette is fine after the first change, but at the second new chain (about 10K miles on the cassette), I'll get skipping under heavy loads unless I also replace the cassette at the same time.