The first thing to try is replace the firewire cables themselves with new ones. In some cases this is all that is needed, the cable connector may lose it's grip over time of many mating cycles. Try that first before doing anything else.
The next thing to look at is the 1394 chipset on your computers. If you've always had this issue, it may because the 1394 chipset was of a lower quality than some of it's competitors. VIA for example did not perform as well as a Texas Instruments chipset, the symptoms you're describing are synonymous with inferior 1394 chipsets. Check our 1394 compatibility list for more information.