I really don't agree with that at all.
You will always want the same pressure (when cold) in your tyres regardless of temperature.
In the snow you definitely don't want tyres which are narrower because it'll be harder for them to bite down. You also don't want a wider tyre in the wet either because a narrower tyre will displace water better.
I'd be very sceptical about anything in that thread, there's a lot of crapola in there like that nitrogen inflated tyres lose pressure more quickly because nitrogen molecules are smaller than oxygen molcules which is rubbish.
http://www.theaa.com/motoring_advice...-nitrogen.html
As for the OP, I'd check the car out. That said is it possible you're just feeling a reduction in lateral grip? I find the Fiat bouncier around certain corners when it's colder.