3com cable modem problems

I just bought a new 3com external cable modem. Its the weird wedge-shaped modem with green lights on the front.

Anyway, I tried running a 75' cat5 cable to connect to the modem. I did an ipconfig /release, renew and about 3-4 minutes later receive an ip from my isp (way too slow). However, I was unable to access the internet. Its obvious I am timing out. Does anyone know if there is a length limit for cat5 cable on this modem? I hardly think 75' would cause it to timeout. I tried this on both a 98se and 2k boxes.

Also, I purchased a linksys cable router, and the modem will not drop an ip to the linksys. The linksys function perfectly on the LAN side, but on the WAN side, I get no ip info. The modem works fine when I connect my pc straight to it with a 10' cable.

Any idea if this modem just sucks or is faulty? My sevice provider is optimum online. What cable modem do you recommend?