#1 What is the exact brand, model, and ver of your router?
For example: Actiontec, MI-424-WI, Rec C.
#2 What is the brand and model of your wireless NIC?
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the speed that you are mentioning.. (54mb, 36mb and such) do not really refer to the speed of you connection. At least not directly.
Those numbers reflect the signal strength. For example 54mb means that you have full signal strenght and you can operate at the full speed capible wirelessly. 36 means that you only have 4 bars of strength, and 2 means that you are barely connected.
your router needs rebooting
you have some kind of interfearance from some outside source.