I do not have said adaptor, but I just purchased a mini displayport to vga adaptor and came across the following...
on Apple's website the customer reviews for their mini display to vga adaptor were very poor (I believe it was rated 3 out of 5 stars). It had equally low scores on Best Buy customer ratings. Apparently, it does not work well in all situations.
Possibly, the same is true with their mini displayport to DVI adaptor. So, my first inclination is the adaptor and not your MBP. However, you may want to scan the Apple, Best Buy, Amazon customer ratings to see if this is a common problem for that adaptor.
Ironically, the higher rated mini displayport to vga adaptors were from third party vendors (whose cost was about 2/3 that of Apple). I bought mine from an online vendor for $18 (vs. Apple's $29) with no tax or shipping.