Is it time to sell the the GPS technology specialist? One bank analyst thinks so.Garmin (GRMN -5.67%) stock tumbled by 6.1% through 11:45 a.m. ET Friday after investment bank Barclays downgraded shares of the GPS guidance specialist from equalweight (i.e., hold) to underweight (i.e., sell), and cut its share price target from its previous $181 to$133.With its shares up by 72% over the last 52 weeks, Garmin has outperformed the S&P 500 by a factor of nearly three. According to Barclays, that was too much ou ...