Since the file being torrented being scanned by the P2P Shield provider when this flush occurs seems to be the cause (the larger the file, 2GB, 4GB, etc, the longer the CPU and memory spike as Avast thrashes away to scan the entire thing over again), I'd suppose unchecking uTorrent from the P2P Shield's programs list would also stop this, but I wouldn't recommend that.
You can add particular files to be excluded (but you could not 'want' to do that):
For the
Standard Shield provider (on-access scanning):
Left click the 'a' blue icon, click on the provider icon at left and then Customize.
Go to Advanced tab and click on Add button...
For the other providers (on-demmand scanning):
Right click the 'a' blue icon, click Program Settings.
Go to Exclusions tab and click on Add button...
You can use wildcards like * and ?. But be carefull, you should 'exclude' that many files that let your system in danger.
But, are you sure it's the P2P provider which is checking that file, I mean, couldn't it be the Internet Mail Provider?
If you exclude utorrent.exe process of Mail provider scanning, will it help? (see details clicking 'Settings' in my signature and browsing for section [MailScanner] in the avast4.ini file).