Wireless interference problem.

I am using a D-LINK DWL-2000AP+ Access point and external antenna.

I am trying to link up another building [ boss's house 100m away]
Everytime the antenna points towards the direction of his house the signal
on Wireless laptop [testing] drops to zero and the Access point starts
dropping packets in blocks [connection on LAN].

I've got 2 of everything and repeated the test, same problem.

It looks like some sort of signal is blocking the communication and causing
the Access point to drop connection on LAN.

There is a small electric Pylon 800m away but 50 deg to the left of the
house.
I don't know of any radio, TV mobile phone TX's near by.

What other possible reasons could be blocking the signal ?

Thanks.

Jim