I find that happens when I am sailing deep down wind then go into a gybe.

In the conditions you describe, you can go 30 knots when entering the jibe, so you'll have a 10-12 knot head wind coming at you when you're straight downwind. If your sail is a bit open, you'll get backwinded - the more open it is, the earlier that happens. IN 18 knots of wind, it should be enough to fully sheet in (more like oversheet, actually), and draw the turn out a bit, so your speed drops to the wind speed or below when downwind. In less wind on very flat water, that may not be enough, and you also need to change the way you flip the sail. It's more like a push-pull rotation like in a heli tack than pushing the clew around (or letting strong winds flip it around for you). That's much easier to do on non-cambered sail than on cambered slalom sails.