Hi Bri,
It's meant to do that. Basically when the engine gets to a certain temperature the radiator fan starts to spin, for whatever reason the fan draws a lot of power to the extent it noticeably lowers the revs. To compensate, the solenoid increases the revs so that it balances out.
You could bypass the solenoid... it would just mean your engine would have lower revs when the fan is on. I don't have a solenoid on mine, it's yet to cause me problems, but I don't have a solenoid only because the previous owner of my BIS wired it up so that the fan is manually controlled. If you have a normal BIS, I'd keep the solenoid and run the car as it's supposed to.
So, is your radiator fan not spinning when you notice the revs going up?