Surging Skyline On Lpg

[samuri]

Received an email from interstate that my 97 skyline GTS( non turbo) running on LPG is surging while driving.

No other details unfortunately.

car(LPG) was tuned in December with iridium plugs and has traveled 7000km since then.

What could be causing this and what follow up questions should I ask to try to pinpoint the problem

Edited June 4, 2011 by samuri

[samuri]

It appears the sound while driving is that of the engine surging but the movement of the car itself is smooth and constant.

Could this be a belt slipping?