Bugged altitude cancels. When using AP I enter new altitude then press vertical speed up. New bufgged altitude is 5500. Panel indicates 700 f/min up and the climb begins. Within a few seconds the new bugged altitude disappears. Aircraft is still climbing. bugged altitude no longer displayed. If I do not intervene the aircraft will climb thru original bugged altitude. Re-enter same altitude of 5500. Aircraft continues to climb to assigned altitude. Once again bugged altitude disappears. This cycle will continue. After entering new altitude four to five times it will stay. This issues occurs in both climb and descent modes. Occurs with any altitude chosen. History- Dynon system has been working well for about six years. Garmin GNC 355 connected to Dynon system via Arinc 429. Thanks!