get on (one's) hobby-horse

To frequently or incessantly talk or complain about a subject, topic, or issue in which one is excessively interested. "Hobby-horse" can also be spelled as a single word." Ah, here we go again. Once grandpa gets on his hobby-horse about the government, there's no stopping him! Uh oh, sounds like Janet is getting on her hobbyhorse again about the need for healthcare reform.