Wireless transfer rate drops

Hello

I have a peer to peer wireless network (laptop-desktop), mainly for
internet sharing. On the laptop it is bridged.

Now, when the connection is established it is at 11 MBit/s and the
reception is very good. But within a few minutes it will drop to 5.5,
2 and then to 1 MBit/s, leaving the laptop with a bad connection and
many time-outs.

Can I somehow retain the higher (or at least 5.5) transfer rate
without having to buy more hardware?

Any hint appreciated.

Stefan/Salzburg