Second possibility - a tight link in the new chain, perhaps where it was joined. (A 7-speed chain can be joined with a chain riveter, a no-no with the fancier newer chains. If the mechanic didn't loosen the freshly driven pin, that link will be tight. Perhaps it doesn't straighten fully until you apply real pressure to the pedals and the kinked links jump under very light pressure.
If you have a FW, do the oiling of my last post. (Always good practice even if you see no need.) In either case, run the chain with the bike on a stand or hung by the seat and watch for a kinked pair of links going through the derailleur cage then over the cassette. (This will probably show up best on the smallest cog.)
Ben