Yesterday we went for a few practice instrument approach and on my first approach as I was getting closer to the airport, roughly 2 miles out, my needle (GRT system) start moving erratically from the far left to far right. We moved to a different and nearby airport for another approach, in a way to rule out localizer issue but got the same result. After a power cycle of the 430W, it was all OK. We tried three different airports, including the original practice destination and all with normal behavior. So, I was wondering if anyone has seen such behavior of the NAV or could this be the start of a problem to come. I guess since no other changes were made with the exception of a power cycle of the 430 in the air, I can rule out any cabling antenna issue.