I have been advertising at a low level on MIVA for several years. My traffic has been rock solid steady at 200 visitors per week. Now, suddenly, over the last few weeks it has skyrocketed to over 2500 and I smell a rat.
Can you check your recent referring url stats to see if you can pick up on where the new inbounds are originating from? Hasn't miva gone into the parked domain thing now as well? If so, there may be a couple of sites parked with names close to your niche, which would result in many of your top keywords being on those pages, and if they are being hit with traffic spikes, that could explain the sudden increase?
One way to check the quality of a PPC program is the CTR (Click through rate). If it is unusually high there is something fishy. Looking into my Miva account I could only find the statistics of the number of clicks, but not related number of impressions. Do they hide that information or am I not clever enough to find it on their pages?
Searching for "click fraud miva" in Google I found out:
I am outraged to discover that Miva in Europe is doing with Golog SARL. Golog (www.golog.fr), a French company, has been working with the European divisions of Miva and various European ISPs to direct error traffic directly to paid advertising sites without users clicking on the selected advertiser, which is effectively a forced click and results in click fraud.
At least in one case I could see it happen that an error page was replaced after a few seconds by the page of an advertiser. What I don`t know: Does this happen with the knowledge of the advertiser or not? If they are doing it only for travel and other high volume keywords, there is at least the possibility that the advertiser knows and agrees to those strange practises of redirecting traffic automatically instead of waiting for a human being to click.