A Citation Captain reported an unreliable signal from LAS VOR on the NORTHTOWN TWO Departure off VGT.

Narrative

I was climbing to 7000 FT MSL as cleared by LAS ATC on the VGT Runway 25 NORTHTOWN 2 Departure (NOTWN2.LAS) procedure. As the non pilot flying; I had the #2 VOR NAV set to intercept the 313 radial of the LAS VOR. During the initial climb; I noticed the #2 needle fluctuating as an indication of a weak signal due to low altitude. As the climb continued the needle fluctuated less with no nav flag but still fluctuating with some indication to join the 313 radial. At the same time I was trying to report the weak signal to LAS Departure; the Controller keyed the microphone before I could make the call advising us to turn left for the vectors to avoid terrain. While being vectored; the Controller asked why we were not on the 313 radial and I advised him of the needle fluctuation. He then responded that they have had other reports of similar needle fluctuations. With normal #2 VOR/NAV operation on the previous flights with departure procedures at two different airports and normal #2 VOR/NAV indications on the remainder of this flight. I believe the unreliable signal on the NOTWN2.LAS departure procedure was caused by the VOR itself or surrounding local interference. Experiencing a faulty NAV signal is difficult for a flight crew to determine if it is the aircraft; airborne equipment; or the ground based equipment. I would recommend Citation and other similar aircraft that have an FMS for #1 NAV and a #2 VOR/NAV also have a #3 NAV that is independent of NAV 1 and NAV 2. This would identify a failure of the aircraft; airborne equipment; or ground based equipment/local interference outside the aircraft.