X5 3.0i Poor Start

[salenks]

Hello,

I was wondering if anyone can help me with the issue I've been having.

My 2003 X5 3.0i sometimes starts like it only fires up a couple of cylinders and then a second or two later, the rest of them. This happens occasionally, not every time. I also noticed smell of gasoline when this happens.
I have been having issues with the battery where the voltage drops to 12.3 V and from what I've heard, it should be 12.5 or more volts. This is not the original battery but a Exide 72Ah/720A and same happens with another battery which is Optima 50Ah/815A. Could low battery voltage cause this? Or what else could be the problem?


I added the recording of engine sound to youtube http://youtu.be/Zr1XXcx1YWs.

Little history of my X5:
-105,000 Miles
-Spark plugs changed at 95,000 miles (orignal installed)
-LPG installed (had to remove original battery because of this and place new battery in trunk side compartment)
-Replaced fuel pump with one for Opel vehicles (has 3.5 bars and BMW has 3 bars pressure)

I was told by a mechanic that this fuel pump will do the job but I am not so sure. I will probably get the original in a couple of weeks.

When the mechanic hooks up the computer to the car, there are no error codes regarding this problem. There are some error codes but those are normal when you have LPG installed.


Thank you all for your help,

Sasa