Gitane Mixte - rear wheel issues: is it the dropouts?

[IvonaDestroi]

So I am working diligently on an old 70's gitane mixte frame, (teaching yourself bike repair isn't easy!) and I've had some problems with the rear wheel.

The original axle that came with the bike was bent, so I put on another 27" wheel from my schwinn which worked fine on that bike. I trued it as best as someone whos learning how to true could... it seems straight enough.

Now this wheel is starting to lean sideways and rub against the seat stays. I'm wondering if the old axle bent from this same problem, could it be my dropouts?

It's fine at first, then a couple of blocks down the road the wheel starts leaning sideways and rubbing the seat stay. The first wheel did this too, of course that one had the bent axle. I've re-installed it a couple of times, (quick release on both wheels), and my bike shop roomate also tried with the same result.

The center seat stay on the chain side is slightly bent outwards, although from what I looked up this is commonly done on mixte frames so the chain won't rub the seat stay. It does, in fact, look like if it were built straight the chain would rub.

However, I saw the same model (possibly a different year, but VERY similar from looking at it) at the bike shop the other day and that seat stay bend was nowhere nearly as pronounced as the one on my frame. The guy at the shop said it was probably fine. Could someone have tried to enhance this bend for some reason? It seems unlikely...

there is one spot on the paint on the inside of the stay where it looks like a chain may have once rubbed but it's hard to tell because there's paint chips all over the frame anyway, but the rest of the paint is fine in that spot.

What do I do now? How do you check your dropout alignment? I always think the problem is bigger then it is, maybe it's something simple that I'm doing wrong? over or under-tightening the wheel perhaps?