If Multiplicity suddenly returns to the primary (usually the top right corner) this means the network connection between the 2 pcs had a problem.  Multiplicity will then try to reconnect in the background.

If you are not using MP 1.05 then I would suggest you upgrade to that as we tweaked the code to detect dropped connections.  Please make sure the primary & secondary pcs are running 1.05 as mixing versions will not work correctly.

If the drop happens every X seconds like clockwork then it is likely to be a case of mismatched files (usually every 6-10 seconds when that happens)

Wireless networks may drop due to interference and if you are using a wired network I would check that there is no damage to the cables.  A minor cable fault can remain undetected for months. 

The other thing to check is that the network is not saturated or the router (if you are using one) is not struggling with other work.  Some 10MBit routers have been known to break when connected to a fast internet connection of similar speeds.

Also make sure quick machine disconnect detection is disabled in MP (controlled via system tray menu->advanced user settings dialog)

We have tweaked a future build to attempt a reconnect quickly in the event of a disconnect & only throw the mouse back to the primary if this connect does not happen with 1 second.  This should help I think.

Obviously we have to throw the mouse back to the primary at some point when connections are lost because otherwise you would find your mouse was stuck on the other PC which would be a problem.

Applications which fiddle with the network sockets may well cause problems because most MP packets are time sensitive.  Any delays and the mouse will start to lag on the secondary.  A 0.25 second delay on sending data to a web site is unlikely to be noticed, but will be very obvious with Multiplicity.

If Multiplicity was working fine for a long period of time and then suddenly stopped working correctly then assuming Multiplicity was not updated, it seems logical to assume an outside factor is causing the problem.

Has the networking been touched in any way, have you installed a new driver, or filter driver (say a VPN).  Has the firewall been updated?

Remember to include both the primary & secondary machines when you think about these things.  It may be the secondary is dropping the connection rather than the primary.

Additionally, we should see if your machines have anything in common.  Do you have MP or MP Pro, how many secondary pcs do you have connected, are they both on at the same time and are they both connected via wireless or wired connections, what firewalls are used, what network adapters (if its nvidia then please turn off any 'offloading' features it has).

A large number of machines at Stardock are running Multiplicity and I do all my vista development via it without any problems, so its not a case of Multiplicity is inherently unreliable.  Something is causing the network connections to drop.  Either Multiplicity is dropping it because the 'are you alive check' fails (not sure if synergy has one of those), or because the network stack has detected a problem and has dropped the connection.

Please do not enable quick disconnection.

It is an advanced user feature for very specific cases (basically when you are using the secondary machine under a debugger and wish to regain control on the primary very quickly)  By its nature it will disconnect you if the network delays by even quite a small amount.

scot3818:  When upgrading to MP 1.05 you MUST ensure the secondary is also upgraded correctly.  If it is not then Multiplicity will automatically drop the connection as the are you alive check changed between 1.02 and 1.05.

A fixed pattern of disconnects is almost certainly a mismatched version issue.