Yes we had a period of time where the Garmin units we were supplying suddenly started to become very inaccurate ( not so good for gliding competitions ) and all were the units with an attached aerial rather than the built in aerial type. We replaced the aerials with after market units at about half the cost of the genuine Garmin units with good results.

As I say just sit it on a fence post and then have a look at the trace, if it moves around randomly, then chuck it in the bin and buy another.