Issue with Jobs and Report Execution

From more then a year we are facing issue with reports from Jobs, almost 80% active  or enabled Jobs are getting failed to execute reports. And most Jobs are used by our users. The message appear in a mail box "Report execution was terminated due to its duration. Try to select a smaller report range or reduce tables count."
Most likely it happens in 1st week of the month. In beginning , we were suggested to reduce columns and content of reports and also reduce the number of units in unit group but it never help us anyway.
At same time the same issue appears with normal report execution. We are not able to execute most of the report for a single day duration.
From last 14 months we are facing this issue, we had raised this issue so many times but still we are not able to solve it.