Hmmm just searched tiscali. http://www.tiscalibusiness.co.za/adsl_index.asp

d. Service Cap

i. Telkom has imposed a 3GB (3 gigabyte) traffic cap on the ADSL service. What this means is that after you have used 3 gigabytes of traffic (to or from your PC) your connection will be put onto a limited network shared with everyone who has exceeded the limit. This will still provide fairly fast access to South African sites, but international access will be much slower. Tiscali cannot be held liable for the result of your service being capped.

This may be what is happening.