why does speed vary?

avalanche* wrote in news: snipped-for-privacy@4ax.com:

Wireless LAN speed is dynamically adjusted by negotiation between the end points to try to maximise speed whilst maintaining a reasonable signal quality, hence acceptable error rate.

Variation between 54Mbps and 1 Mbps is excessive if the end points are physically static, and suggests there is an interference problem.

It's either interference from some other 2.4GHz device, or you have an old buggy version of Intel Proset that does that without much provocation. In theory, if the access point detects any transmission errors, it will reduce the speed until the errors go away. If you don't like this feature, you can set the wireless speed to whatever you find amusing in the wireless access point web based setup utility. However, if it really is interference, you'll be sending or receiving errors instead of data.