Oval pattern on disc. Unbalanced drive?

I've been searching the forums trying to figure out what my problem might be but I haven't been able to find any answers, perhaps because I'm not familiar enough with the jargon and vocab to know what search string to use. So I'll just try to explain what's happening and hopefully someone out there can figure out what I'm trying to say.

I'm having problems with the playback of about 50% of the dvds I burn. Media choice doesn't seem to be the issue as it occurs regardless of which brand I use (but I suppose it's possible I just have 2 or 3 bad batches/brands). It also doesn't matter what playback device I use so the problem is with the burned dvdrs themselves. I probably would have just bought another burner already except that the one I have is a fairly expensive Plextor so it'd be a shame to have to replace it so soon...

Here's what I observe. After burning a disc (dvd minus r's recently) I have noticed that some of the discs have a sort of wavy, blurry, ovalish, pattern on the burned surface. This pattern is always nearer the center of the disc, around where that inner circular line can be seen before a disc is burned (don't know what that is, but it's present on all discs). This ovalish pattern always appears more "smeared and blurry" in two opposing quadrants and then more defined in the other two. The discs that display a stronger pattern are always the ones that don't work, and the ones with a fainter pattern or none at all work fine. I can normally tell just by looking at the burned surface which discs will work and which will not.

So my question is, what's causing this problem and what's the fix? I'm afraid this could indicate something like an imbalance in the motor/spindle of my burner, meaning it'll have to be replaced. But at the same time, it doesn't occur with all discs (even when using the same brand from the same batch, some display the patten and some do not) so why it would only happen half the time instead of all the time is a bit puzzling. The discs I burn are almost always full, so it's not that some use more space than others. I just don't know enough about the burning process to know what causes such a pattern and what it might mean. (I'd be very curious to know though.)

For what it's worth, the brands I've been using recently have been Verbatim, TDK, and GQ. It happens despite disc type and speed. Also, if a burn is unsuccessful once the same image is just as likely to burn ok on the next try. I also never put any labeled or unbalanced discs in my drives. I was just about to purchase a new burner when I thought I'd come here first to see if anyone could help. Thanks a lot.