"Your original notice is correct (at least for AppleShare and the Ethernet driver, I don't know about it's effects on TCP/IP). There is a problem with the Ethernet driver and PCI Power Macs. This problem, it turns out, existed in 7.5.3 and 7.5.5 (and probably even further back), but it was made worse in 7.6 (although it probably is still fairly uncommon). There is considerable effort being put into fixing this, but it is not fixed yet. For those who have updated to 7.6, the suggestion is to copy the Ethernet (Built-In) driver from a 7.5.5 System. This reverts the problem back to its 'rare' condition. This problem was only discovered in the past few weeks."
Update: On today's MacInTouch, Ric links to this item (thanks, Ric) and covers other recent reports of network problems, including the "slow network" problem reported by YARC (and mentioned here previously).