iMac wifi connection keeps randomly dropping

Hi

I've searched many threads, other sites to no avail.

I have a mid 2011 27" iMac running 10.8.3 (i can't download 10.6.4 as it keeps dr

My wifi hub is a time capsule 7.6.3

On a random and regular basis my wifi connection drops - often within seconds of me forcing a reconnection.

To date I have tried:

- set up a new location

- changed the wifi channel

- altered the MTU (is that the correct tla?)

- turned off time machine

To no avail.

I can use an ipad beside it and it never ever drops the connection.

There is no warning it has dropped - everthing hangs. It always reconnects when I run network diagnostics.

Any advice appreciated