If you are running your fan thru a switch w/o a relay, it's a disaster waiting to happen. Sounds like it's started. First, check the voltage at the fan while running. It should read close to battery voltage after startup (12-14 VDC). If it does, the fan needs to be replaced. If the voltage is reading lower than that, like less than 10V, your fan switch wiring and/or switch is deteriorating and impeding the full 12V+ to the fan, thus lowering the fan speed. If this is the case, you need to rewire the fan thru a relay which should have been done in the first place. There are plenty of threads here to show how to wire a high amp draw device, like your fan, using a relay.