I was plagued by this situation a while ago, and still a current problem, although the solution I hope will be quite an easy remedy, change the tyres!
The car came with Indonesian cheapo's on when I got it, and according to one of the tyre place chaps they are quite old and not perfect, so when I get some money I will change them. Had all the tracking sorted etc, balancing etc and they told me it was straight, my steering wheel is still at 12.15 not 12pm, but when the new tyres are fitted I'll get it all checked then.
Had lots of people look at it, including the Council's MOT people when it had the thorough MOT this month, which it sailed through, and even got complimented on, not bad for a daily run around!
Might actually have to spend some money on her and even get a service?

If I did have the money, I would do a total refurb, back to the original everything, but at the moment money dictates a little at a time, tyres next as there is no grip at all in the wet!