CRJ-900 Captain reported an altimeter malfunction resulting in an altitude deviation during cruise flight in RVSM airspace. The Captain switched air data source and continued the flight to a landing.

Narrative

During flight on multiple occasions; the First Officer (FO) (Pilot Flying) side altimeter setting kept going on its own. Once in the climb. A couple times in cruise and once on descent. It seemed to occur when the air data reference panel was used for another purpose besides the altimeter setting; such as minimums and v speeds. At one point in cruise it went hay wire and set itself to a high altimeter and the plane started to descent. In RVSM airspace this happened and we got a traffic advisory below us. We descended about 200 feet before the descent was reversed and got within 800 of the plane below us. Neither the other plane or ATC said anything about this and we corrected to the altitude assigned. No further such incident with traffic. Each time this happened in flight I immediately asked the FO to check the altimeter setting. When it first happened in the climb I assumed it was him accidentally turning the wrong know. It wasn't well communicated to me that it wasn't him. The second time in flight in cruise the same thing. The time we had the TA in cruise it was finally communicated to where I understood it wasn't him; but the plane. During the descent I watched it myself jump around without anyone touching it. Once we noticed it was indeed the plane that's when I switched the Flight Control Panel (FCP) to my side and transponder. It should have been sooner but we didn't know it was a plane malfunction at that point.I would believe something is wrong with the FO air data reference panel. I should have switched to the captain side on the FCP and transponder (for RVSM) but we eventually did this later in the flight. And disconnected the autopilot early so the plane wouldn't do anything crazy close to the ground. The first 2 times this happened I was under the assumption it was the FO accidentally using the wrong knob. The first time it happened I noticed it within 1000 feet of level off and my altimeter wasn't at the alert point yet and I immediately said check your altimeter". This is one of those random events and need to know systems. Knowing the altimeter was off was key to preventing further deviation."