TCPDUMP and "Bad cksum" errors

[isam]

Help! Over the past couple weeks ive been noticing declining conditions in the speed recieved over my cable modem. Before, I was getting around 4-500K/sec on average, now the most I seem to get is around 330K

I have tried communicating with the network tech at my isp, and he said that in order to further trace the problem, either he or I should attempt to run tcpdump to further ascertain more information about the problem. I have done so, but I have noticed that I seem to get this :

23:39:12.459888 192.168.1.100.49795 > 209.123.109.175.80: . [bad tcp cksum 5d78!] 1:1(0) ack 1 win 33304 (DF) (ttl 64, id 26354, len 52)
23:39:12.460010 192.168.1.100.49795 > 209.123.109.175.80: P [bad tcp cksum e605!] 1:500(499) ack 1 win 33304 (DF) (ttl 64, id 26355, len 551)


when downloading files, but not when uploading. I am not sure what to make of this, other than there is a problem.

So, is there anyone out there that can help please?

My Specs:
Powermac dual 1.25ghz
OS 10.2.6
1 gig ram

Further information:

Linksys router, 4 port, with a pc and 2 macs (laptop and desktop hooked up to it.)

Ive tried plugging the cable modem directly into the desktop in order to figure out if it is the router that is in some way causing the problem, but got the same thing. I've tried resetting the modem, renewing the DHCP lease, etc, but I am unable to figure out what the problem is.

I'd really appreciate some advice if possible

thanks in advance