Why Is My Ad Bouncing Around?

My ad has wide fluctuations in ad positioning

[metakomm]

I have been running an order since march and pretty much hold down the #1 - #3 spot with 30-40% CTR...

Anyway, I notice every so often, including today, when putting in one of my best keywords and refreshing to see where my ad will show, each time i refresh the following will randomly happen:

1) My ad won't show
2) My ad will show in the #1 spot
3) My ad will show in the #7 spot
4) My ad will show in the #6 spot
5) My ad will show in the #4 spot

I could understand bouncing around between #4 - #7, but to go from no ad shown, to the #1 spot, to the last spot on the page with a high CTR?

Like I said before, I was always in that top position...I guess it is possible a bunch of people have set a much higher max CPC and may be throwing things off a bit...