We are now up to over 13,000 queries passing from Cognos 8.4 to our Microsoft SQL server. We are seeing where SQL shows SPID resources from Cognos, but Cognos does not show associated background or interactive activities. Periodically when this happens, SQL will come to a crawl (No CPU or Memory resources being used on the SQL server). We can kill some of the "abandoned" SPIDs and get things running again.
8.4 had issues with Cognos processes. After upgrading to 8.4.1. we never had SPID issues again. Sorry to be so non-specific, but the first 8.4 version was not what it should be..