I have a 98 A4 quattro with a boatload of miles on it (about 230K miles - 370Kms). I do require an alignment and have put new tires on the back without aligning yet. I drive 60 miles a day. Suddenly in the snow the other day the car started fishtailing, like the front and rear of the car were driving separately. I checked the tire pressure, but fronts were only down a couple psi. I was getting a slight "tuck in" feel from pass rear when I took sweeping left hand turns entering the highway at about 50 mph. I plan to check the wheel lugs and bearings in the morning. What surprises me though is the suddenness of the weaving. Worn inner CV? Worn pass rear bearing? alignment? Blown strut? Why so sudden? Didn't hit anything, and was doing 80mph on the highway (dry) only two weeks ago.
Whatever needs fixing, I will fix myself. Probably take to steering specialist when I have a chance to verify what problem is. Anyone want to suggest most likely culprit.

 

Depending on the "type" of snow its not odd to be a little loose on snow at 5-80 mph. If you experiencing sudden swings of the car check the tie rods, and all the ball joints before taking it to an alignment. Have you replaced the control arms? My car felt sort of like your describing when my control arms needed replacing.

 

I knew that I had an alignment problem as both of my rear tires were wearing stongly in the inside (I replaced both tires) and now see that both coil springs are cracked at the very bottom. So a) alignment needs fixing anyway and b) no sense til I put new springs in. Could be something else too. However yesterday was wet (but no snow and ice) and the car handled perfectly. However when I hit a small section with some ice (and where many trucks drive=oil on the road) the front and rear of the car acted almost independently...horrendous fishtailing, then fine again. It reminded me of a 70's Corolla I had which the frame broke on both sides (rust) and the entire front end was separate from the rear. I am guessing that when a tire slips and drive force changes more to front (or whatever) the sudden shift in drive accentuates the misalignment. When dry, the car tracks absolutely straight. This also explains (I think) why I was feeling a slight "tucking in" of the right rear when I entered highway flyover sweeping left at 60 mph, I figure rear coil was bowing too much due to broken coil. 230K miles, so I will check all at steering specialist also. I've had 30 cars, and I have (except the Corolla) never been so suddenly "sh&* scared". Awesome still on corners, up hills etc.