Obviously we need more information from you in order to even begin helping you...
1. What version of the game are you running?
2. What O/S are you both running?
3. Are your PC's closely matched in hardware specs or is there a rather large discrepency?
4. What if any security software did you have enabled/disabled during this LAN-game?
Since the DEVS are aware of a "desync" issue with LAN/MP games please help them out in finding the solution.
If you have a savegame or replay saved please email same to support@ironcladgames.com
thanks,
the Monk