
Two shots at it: (1) Check the tires again - if you can see the wear bars,
you're down to problem area (and some tires pass a penny test between the
bars - that's no guide at all). Your problem in the wet is call hydroplaning.
You may not have enough tread left to channel water out from under the
tire - so it goes fishy on you as it lifts off the road.

(2) Tires age. In particular, soft tire compounds get harder as you put
them thru more heat cycles. Harder compounds don't grip as well as
soft ones. Effect is very noticable on tires that get very hot very
often, such as in competition, but it hits all tires.