tesla coil interference
from http://www.pupman.com/safety.htm on Tesla coil safety:
There seems to be room for more experiments: Maybe chart the radiation reading vs. distance to the Tesla coil, shield the GM tube with grounded screen or thin metal to see if it is RF (this wouldn't stop X-rays, you would need lead), and try to see if it is coming from the fluorescent tube or the coil gap.
...any time the voltage gets above 10 kV, there is a significant risk of X-ray production, and the risk increases with increasing voltages. ... If the potential difference between the anode and cathode is +100 kV D.C., a spectrum of X-rays will be produced with energies from zero to 100 keV.