Hmm sometimes i still lose fuel pressure, even though i didnt got hit, neither overused WEP, hardly used it at all actually.

Why could this problem be?

It could be a thing called Vapour Lock.
http://en.wikipedia.org/wiki/Vapor_lock

Hmm i dont fully understand, but I can also say i didnt go that high.

Ahh ok. Does your engine cut out or splutter when doing negative G?

Hardly do Negative G's hehe
But its probably some manuever thing, or still Engine abuse even without going into WEP. It was 40 mins into the game.

yes 40mins of furious flying is enough to kill the engiene in the original so it might just be your pushing the aircraft to hard!

I don't know if this factor would be included in the game, but I believe that some aircraft relied on gravity to supply fuel to the engine. IRL this was an issue during inverted flight and other acrobatic manouvers.

Yes, that is the case for the spitfire. The 109 did not have this issue.

As for throttle, it's like I said, keep it in relation to your oponent's speed(don't go so fast that you overshoot and do go so slow that you lose him).

As for the rudder question, tight turns kills airspeed anyway. The rudder contributes just a little bit.

Pretty much yes - The Merlin engined spit and hurricanes used a float type carburettor - basically a float valve similar to a ball-cock on a toilet cistern which in positive gravity maintained a reservoir of fuel to the engine, under negative G it simply shut the fuel supply off temporarily. The 109 had fuel injection which was unnafected by G. Whist the Merlin had the disadvantage in this respect the carburettor fed system gave it a bit more 'grunt' compared to the fuel injected Daimler Benz.