One of the adjustments going from groomed to powder is to use less edge in powder. It sounds like you've got the opposite problem: riding the skis too flat on the groomers. Try tipping them more into edge. One of the knocks against wide skis vs narrow skis on the groomers is that wide skis take a little bit longer to go from edge to edge. If you're compensating for that little difference by helping the skis turn, then they will get squirrelly.