Yep, that understeering in a straight line is a bitch.
Having run every combination of tyres in the known universe, from brand new RE050's to tyres on the wire, I would personally say your tramlining is not related to the grip of your tyres. It may well be related to the inflation, or your cars alignment, Id be checking them first.
However, Ekona is right that you need to be considering changing the fronts, Id certainly make sure you have them done before actual winter starts.