cellfone’s GPRS connection

Hi guys,Lots of commecnts here but my problem is kinda off topic here.i connect to internet from my PC through my cellfone’s GPRS connection.But most of the time speed is really very slow i cant opend any page it takes forever to open, but sometimes its very fast,faster than my dialup, i get download speed of more than 48kbps. so can any1 plz tel me wots the problem why there is varition in the speed.most of the time its slow.can somebosy help me?