We suspect that you are running into packet routing problems being caused by a
remote AppleTalk bridge somewhere on the Ethernet. If there is a slower-speed
link somewhere in the Ethernet (perhaps a 9600-baud link between remote sites),
and, if that remote site has an AppleTalk bridge anywhere on it, you can get
the situation you describe.
The problem arises because AppleTalk is treating the Ethernet as a single
AppleTalk network, even though the Ethernet may consist of many networks in a
complex setup. Because of this, the Macintoshes directly connected to the
Ethernet use whichever bridge on this network was the last one to send an RTMP
(Routing Table Maintenance Protocol) packet. Even though the VAX running Pacer
software and the Macintoshes are on the same physical cable, a Macintosh cannot
send its keystroke packets via that remote bridge, because it has no way of
deciding which bridge is "closest" in terms of actual transmission time.
This does not happen when you put a Kinetics FastPath between the Macintoshes
and the Ethernet, because the only bridge the Macintoshes get RTMP packets from
is the FastPath, and it knows enough about the Ethernet to properly route the
keystroke packets directly to the VAX.
One possible solution is to have the Ethernet bridges on the local side of the
network refuse to transmit AppleTalk packets. If this is done, then the VAX
running the Pacer Software once again begins acting as its own bridge for the
Macintoshes directly connected to the Ethernet. The two packet types that you
want the bridges to exclude are RTMP (Ethernet type 809b) and AARP (Ethernet
type 80f3). You need to consult the documentation for your bridges for
instructions on how the bridge excludes packet types from retransmission.
Be aware that stopping these packets from being transmitted over a bridge keeps
you from using AppleTalk over the remote link.