Based on your description, I doubt it's the switch but you never know.. When the switch is on, it may have a poor connection and generate lots of interference, kind of like a miniature spark-gap transmitter. The fluorescent bulbs are known to ruin cheap light switches. It makes the switch arc a lot when the bulb is switched on and off. Also, any inductive loads, like motors, can cause spikes in voltage due to self-induction when the load is turned off. Usually a bad switch will generate RFI when the circuit is on. It also has RFi spikes when the load is turned on and off. I doubt the switch is the problem if the noise is lower when the switch is off.
You problem may also be caused by another device in the same circuit, like a dimming light switch or a pulse type (switch mode) power supplies. Turning the light switch on, creates a different path for common mode currents thus reducing the noise. Try to find out what else is on the same breaker and try to isolate the problem. The RFI noise may get to the radio through grounding (if any ) or the power circuit, as well as the coax shield.