Starnge behaviour between wireless points

I have provided a belkin fsd7132 (range extender) as the main AP in our office and configured another of the same in a remoter building as a relay (followed the instructions). Works mainly OK. I installed another PC with a belkin PCI card in the remoter building, this is further away from the other relay and the signal was too weak for a reliable connection (keeps dropping out). Use channel 6 to avoid other local networks.

I then installed another relay point closer to the PC to improve the signal. This seemed to cause the laptops using wireless in the main building going through the main wireless router to drop out. When I swithched the last relay off this seemd to restore OK.

What happened here? Can I have multiple relays?

If I decide to run a cable between the buildings to extend the wired ethernet, what system should I use at the far end to get a better signal in that building?