IMHO of course, if you have checked the tire pressures and they are normal then the next logical step is to have the alignment reset to spec and the suspension parts checked for wear (the bushings). I would use a good Porsche Indy for this work -- the dealer will just say its the non-N rated tires.
We have Sumitomo HTR ZII on 02S and ran through a 2" downpour with no problem at 60MPH. IMHO, I would say its not the tires.
Take care,
Ed