3Com Router

I have this 3Com router
http://www.3com.com/products/en_US/d...=3CRWE754G72-A

It has 4 ports on it and I have my desktop and notebook plugged into the ports
and they are working ok, I also have another cable in another port which goes
across the other side of the workshop where I do repairs and plug in pc's to get
internet access and networking access 90% of the time it works ok but some pc's
I plug in tells me there is no network activity and I try to release and renew
IP address and nothing works the only thing that I have to do is switch off the
router and on again, is this normal behavior?

Thanks