If you see poor conversion on one engine, but the landing pages look good, products haven't gone out of stock, and very tellingly: other engines have not experienced a similar dip in CR, here are two data dives that can be revealing:
1. Performance by Domain to see if there are domains that are bringing in poor traffic. (Ask.com is riddled with poor-performing syndicated partners.) I noticed that ave99 was bringing in a bunch of traffic on Yahoo for many of our client, with a few to no sales. (Here are a few other sites on Yahoo to watch out for: couponmountain.com, toseeka.com, shopica.com, findstuff.com, freedownloads.com, landing.trafficz.com, webmd.com, addictinggames.com, thefreedictionary.com)
Yahoo and Ask.com allow us to block domains on the search network. Google has yet to offer such an options in search.
One thing to keep in mind when chasing down the 'problem', Yahoo and Google discount CPCs based on partner site performance, so maybe this is already being taken into account through reduced CPCs on these sites. Because we are not able to see a line-item report of discounts, the trouble-maker domains getting a discount are a mystery. I dropped a few big stinker sites on Yahoo and actually saw an immediate dip in the cost/sales ratio, suggesting that these sites are not being discounted enough to justify letting the KWs serve on these sites.
2. Also, you can look at the Search Logs to see if the engine(s) are being too broad in their matching games. Are the engine serving a "flowers" ad for a search on "gift baskets"? This would be a problem because the landing pages (and copy) are not tailored to the actual search query. You want users to land on a page where the content matches the search intent. (It is best not to give users more options than they asked for, but also, not to show them only one or a few choices if they searched more broadly.) You can change matchtypes or add negatives to cut some of this poorly-matched traffic.
Tightening up the traffic will lift conversion, which means you can afford more per click on domains/keywords that actually convert!