As per post above, possibly the starting transient of the fluorescent is triggering the PIR spuriously. This could indicate excessive voltage drop on the circuit, or that the PIR is unreasonably sensitive to mains spikes. Either try a different PIR, a different starter (because the suppressor in the existing one might be duff) or possibly better still, fit an electronic 'Pulsestarter' in the fluorescent in place of the glow starter.
E2A You can confirm that this is the problem and not a wiring error, by first removing the starter. If the PIR now works normally, it's probably interference from the florry. If it still malfunctions, it's your wiring.