A3 8P 2.0 FSI Starting Problem

[squerl]

I'm looking for some help as to what could be causing problems to start for my Audi A3 2.0 FSI 110Kw 2003.

I've been to mechanics four times now for this exact problem yet it's still there.

The problem:

Car starts fine on cold engine / first time. After that, the following starts are hard. It doesn't matter if the engine is cold or hot. I can start the engine, turn it off and the second try it won't always turn on. The starter is spinning, everything sounds "good". After leaving it for approximately ~20minutes it starts okay. 

Fixes applied:

New battery. New generator. One mechanic suggested it could be "a piston position sensors" preventing the start as to not damage the engine. As replacing that sensor would be costly, he first replaced some others (have no idea which ones). Yet the second mechanic said that the computer didn't show this problem. 

A friend suggested it could be fuel pump's computer, after relying this message to a mechanic he said he will look in to it. He replaced it with the one I got when I bought a car(was left in a trunk). I told him to make sure it works, but he just replaced it. 

The other mechanic after diagnostics said I need to reprogram engine control unit, which he did. And it fixed it for some time, but then the problem came back. The same mechanic later said that the problem might also be caused be the aftermarket car alarm, so he uninstalled it. That didn't do anything. Later, after complaining he said "we fix what diagnostics shows us".

I don't know if this correlates with this problem, but my fuel consumption is showing so wild numbers, like 20-30l/100km. And the gas level is also jumping. It can go from empty to full in a matter of seconds. (This problem was present even before replacing the fuel pump module)

Thanks for any suggestions. Though I'm not a mechanic I was thinking maybe it's the spark plugs (idk when they were changed, I bought a car 2 years ago). As the starter sound good while spinning it's just there's no "explosion" to start the engine. I know this might sound stupid but again, I'm not a mechanic.