Deluge no download issue on pi

[maddy_in65]

I use my raspberry pi as NAS and torrentbox. Deluge sometimes stops downloading for some time it may be 1 min. While in not download state it is still connected to peers but not downloading. I tired different torrents but deluge stops downloading data after few mins for about 1 Min and it again resume downloading. I tried googling about this issue but cant find concrete answer. BTW i download data on external 500GB HDD and it is connected to pi using powered USB hub.I updated deluge 1.3.3 to 1.3.6 but the problem is still there? What is the problem?and has anyone facing this problem?