I would think that if your problem was with the power steering, it would just get stiffer in the cold, but not necessarily "lumpy".

Put your front wheels in the air. If the lumpiness coincides with the steering wheel rotation (ie. it stiffens up at the same point on the steering wheel on each turn), then your problem is likely with the lower steering shaft universal joint. If so, a bit of penetrating oil (PB Blaster) sprayed on it might remove this symptom for a while, but you're probably looking at replacement, ultimately.
--
David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto

Dear Bara,

May this find you well. Cold stiffens rubber: in extreme cold (-60 deg Farenheit or lower, rubber items get so brittle they snap). Thus, if your power steering belt is nearing the end of its working life, cold may cause it may take a "set", when the car sits. When you start it, the belt needs time to warm up, and so to regain flexibility. Inspect the belt. I'd not be surprised if it shows signs of age: cracks, etc. If the belt is cracked, replace it. This is not a costly or hard thing to do.

You might also want to check the power steering fluid level. If it is not at least at the "MIN" mark, add Dexron III Automatic Transmission Fluid, to bring the fluid level up to the "MAX" mark.

Hope this helps: Have a Happy New Year!!

Yours faithfully,

spook