336x280 is usually one of my best performing ad units and I have it set up for text/image. Today I'm noticing a lot of image ads but they are 300x250 size. It seems there is not a lot of image inventory for the larger ad unit, so they are showing smaller image ad instead of text.
Today has also been very CTR on my 336x280 ad unit. Revenue less than 50% off average.
Just switched 336x280 to text only and will see what happens.
Seems if you have text/image 336x280 you should get that, not a substitute.
I've never seen 300x250's substituting for 336x280. That's a bit odd.
I gave up on the 336x280s though, as I had better luck with the 300x250 size. I don't know if that's because there are more advertisers or what. But I also like the fact that they take up less space on a page. Works better for me.