I have finally discovered why Boinctasks tends to stall (in discussion with Boinc programmer Vitalii Koshura). It's the fault of AVG, the virus checker! This running on all my computers with Boinc, it interferes and keeps scanning the files for Boinc when it doesn't need to (I've complained to AVG). Hence Boinc does not respond in a timely fashion to Boinctasks. I fixed it by creating exceptions in AVG to not touch files in c:\program files\boinc and c:\programdata\boinc
Thanks for reporting, this should help other users.