I have circulation system on my water heater.
First system gave out.
When taken apart, it was clogged with sediment and couldn't be cleaned.
Put sediment filter on line, and second circulation system works fine, and timer works fine too.
I haven't had timer problem, but have investigated timer motors running fast and slow.
My friend who re-winds big pump motors says homeowner timer motors are engineered for 60Hz.
As long as electric power is 60Hz (60 cycle AC), then motor will keep exact time.
Since your household current is 60Hz and other electro-mechanical timers run correctly, then it's likely timer motor has a flaw or has gone bad.
Grundfos water heater re-circulation pump and timer are sealed unit.
Grundfos manuals do not show parts list or repair steps:
http://waterheatertimer.org/Water-heater-recirculation-system.html#GrundfosIt's aggravating when expensive household timers like Malibu power packs and Grundfos pumps are not made to be repaired.