Ecopia 422 (15") question

So I put these on about 5 weeks ago and for the most part I am extremely pleased with the fuel economy boost (about 6 mpg) over the worn out oem's they replaced.

I can confirm the squishy sidewall feel when taking turns at speed. It's annoying, but tolerable. The biggest issue that I am trying to address is on the freeway. Around here, various parts of the freeway are grooved concrete sections with the grooves running vertically ( the direction of travel ). When driving over these sections at say 70mph it's similar to huge crosswinds blowing you side to side and requires ongoing correction to keep in your lane. It's a strange sensation.

This is only a problem on those particular grooved sections.. Is this something that could be mitigated with a change in tire pressure? If so, what pressure would you think? Presently running at 40 psi all around. If this isn't correctable, anyone know what Costco Tires return policy is? I picked up Bridgestone because it was the coupon at the time and $400 out the door was too good to pass up.

Any Suggestions much appreciated.