Intermittent upload speed; cycling on and off

Does anybody know what can cause a SmartFTP v2.0.1.1001 to cycle intermittently on and off on uploads? Is this SmartFTP or one of the ISPs in the middle doing this?

Here's a link to a jpg snapshot of the monitor:
http://img2.freeimagehosting.net/image. ... e49961.jpg

Thanks,

UrDaddie