The power on one of our circuit serving the garage and outside outlets goes off, but the circuit breaker does not trip. It used to go off for a few minutes/hours and to come back on for days/months. Now it is seems to be off most of the time, but still comes back occasionally for a few minutes without us doing anything. There is pretty much nothing connected on the cirsuit currently, except for the garage door opener. I checked all switches and outlets and wiring is fine. There is a bathroom GFI on the circuit, but there is no power at its input, and I did reset it just in case.
Can this be caused by a defficient circuit breaker?

Circuit breakers don't usually go on and off. The garage and outside should be protected by GFCIs. Yes? You could have a loose wire in the GF that controls these circuits. Vibration from banging doors can make the wire have momentary contact. Check all the wiring at the outlets.

There is no GFCI outside or in the Garage. I can replace those outlets, but I checked the existing wiring and it all looks fine. The lack of GFCI couldn't explain why it goes on/off, could it (assuming there is no wiring issue)?

I don't know where you live or how old your house is but outside outlets and garage outlets have been GFCI protection required for a number of years. They do not need to to have GF outlets, just protected by a GF. I'd do a little tracing to see if they are not connected to a GFCI protectee circuit. Please explain what you mean by "..bathroom... GFCI but no power at its input"? Could be something there.