I had something similar happen a couple of years ago, except I had a slightly different setup - Netgear router and 3Com OfficeConnect gigabit switch. Three PC's, two servers & printer plugged into the 3Com switch. One day the wife starts complaining about lousy internet speeds. My PC and servers OK. A few days later I started seeing the same problem. Reboot things, seems OK, then over a couple of days goes west again, to the point that it's unusable. Replugged things into the router and presto! All good. Replugged back to the 3Com switch and once again speeds are woeful. Replugged back to the router and gently retired the 3Com switch to the recycle bin. I wouldn't mind betting your OfficeConnect thing is sick.
If your neighbours don't listen to The Ramones, turn it up real loud so they can. “We didn't have a positive song until we wrote 'Now I Wanna Sniff Some Glue!'” ― Dee Dee Ramone "The Democrats want my guns and the Republicans want my porno mags and I ain't giving up either" - Joey Ramone