You need to actively watch the partnerships that these PPC's have with the big engines. The reason you did well before with Overture is because they were listed on AOL/Earthlink. Since then, those companies have moved to Google listings and as such traffic volume has dropped quite largely. To try and keep customers, the PPC's have started getting new partnerships with lower quality traffic such as displaying listings on type-in domains (an example is
www.wiredsex.com), where someone doesn't actually do a search but is shown search results from type-in traffic. This results in lower quality conversions.
This explains the result as to why SE quality has dropped on some of the larger PPC's...
WG