We also have half USR External Sporters, these are cheap, but seem to
have the best connections. You must have an external fan to keep them
cool. We have them stacked on their sides inside a rack with woodin
shelves. Some companies sale rack enclosures for the sportsters one
such company in france, whose name I can't recall and haven't been able
to relocate in the last 5 minutes, has a nice product for a great cost
with or without modems. It is worth checking into if you want
Sportsters.
You may want to try an external fans on your current modems to see if
that helps. The more the better.