Inconsistant network data transfer rate

We have a peer-to-peer network with 12 workstations (95/98)thru a router. On some PCs the following occurs:
We work with access Db (and other large files)over the network with very acceptable speeds, however this may instantly slow to snail pace. Network activity light shows a slow steady blink. Other PCs access data fine. This continues thru many reboots then weeks later returns to full speed, only (weeks later) to slow to snail pace again.