Okay.. you said its running on cable.... try connecting a fresh install PC directly to a PC (this is to get rid of anything which could be interfering with the process).. just using whatever is recommended with your ISP... if the problem still occurs, it suggests either a faulty ISP end-point (ie, your cable modem) or a problem with your ISP......

If the problem doesn't occur when directly connected to the cable connection, it suggests a problem with the gateway (ie, the PC sharing your internet) or the equipment its travelling over (ie, hubs/switch)....