Tugs or tensioners is not for your bike. All I've seen have required rear-opening dropouts. Doesn't bode well that the mechanic suggested them.
First, check axle protrusion. The axle should end flush, or a little below the outside face of the dropout. It has to for the q/r to be able to clamp down properly.
Next is check locknut and q/r condition. The surfaces that meet the dropout should be nicely serrated. If they aren't, replace. Next, check dropout condition. Scrape off paint from clamping surface.
Oh, I almost forgot. Check that the centering springs on the locknut aren't playing up. Either remove, or check that they're installed pointy end inward.