Cable that is too long increased network latency.  You need a
reflectometer
to measure that.  It is very likely the source of much trouble.

The only protocol that I know that could go that distance is either the
"long
cable" spec supported on some 10baseT  hubs (but meant for
backbone/hub to hub connections), or 100VG-Anylan.  Is the cable
CAT 5? I don't recall just what qualifications and details 100VG puts
around the longer cable.

While 100VG isn't too popular, it would appear to be a good fit in your
situation and allow you to put off new cabling - you might even get some
good deals in the used market.  Be sure to buy HP NICs, so you get
good driver support and it will continue at least for a while.

Richard Gambrell