Hi ,
The first thing is if the jobs are failing intermittently then please verify at what time & when the job fails is there any known behaviour also while doing the exchange backup the online defrag on exchange should not be going at same time if that is the case please schedule it to run at some different time also make an exception for antivirus were you backup job are going to like to which backup to disk folder & also make exception for C-temp on media server
2 Also you can try making new account & add it in domain admin & administraotrs group & also give exchange administrator by going to system manager for exchange & right click on organization to click on delegate control & give the right mention above & then make a mailbox & activate it by sending email
3 Then add that account in network log on account & then try to backup see if that works fine is that stable or it is intermittent
Thank You
