Regional Jet 200 ER/LR (CRJ200)
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Distraction
Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Aircraft; Human Factors
During cruise; we noticed that the altitude preselect bug would randomly jump to a new altitude without us touching the altitude knob. This happened one or two times; and then stopped for 20 minutes or so. On approach into LGA; we were assigned and selected 4000 feet. Right before altitude capture occurred; the altitude preselect jumped to around 50;000 feet or so; and jumped up and down in that range; settling at around 46;000 feet. As such; we went through the assigned altitude since altitude capture did not occur; stopping our descent at about 3500 feet. As we were correcting back to 4000; ATC told us to maintain 4;000. We told them we were having an autopilot issue; and they asked if we needed assistance. After this point; I hand flew / used the 'altitude hold' function. No further incident occurred and no phone number was given to us.My main error was that I looked away from the FMA as we were nearing altitude capture. I had made sure to closely monitor this whenever we were approaching a new altitude after I saw the altitude preselect jumping around on its own; except this time I was looking away for too long and at the wrong time.
CRJ-200 First Officer reported an altitude preselect anomaly led to an altitude overshoot on arrival into LGA.