Have you checked your logs to see if any errors are reported, sounds like it might be temporary memory, but that it just an educated guess, which is why I am saying check your logs...
Btw - good report design - would you try and eat a banquet on a single plate?
No you would serve it manageable portions!
Should you replicate a business' legacy reports as is just because that is what they have always done?
No - to get the best out of BI report based communication to the user should be focused, think of a car dashboard and how it tells you at a glance, rather than a 747 cockpit where you might miss the red light amongst the 'noise' of all those other columns...
Please check the logs, get the physical query generated and try to run the same on the DB.
Please verify if both the queries (correct and distorted) return same results?
Also, please try to disable cache and then try again to follow above steps.
This will surely give you the bottleneck and help you progress.