Start problem

Hi,

i've a big problem ..but let me explain

I've a 1.6 My05 Tiburon with LPG and I have a problem : when I try to start my car after some hours that is parked I fail the first attempt ...

and after the second attemp if I accelerate ...the gauge meter goes down like if I have some air in the motor..for some second..
and only after some minute I can use my car

give a look here if you can


Spark and spark cable are new
LPG is ok
The alarm light it's for and O2 Sensor Alarm