SOLVED! Thank you Whitestripes!
Okey so as I wrote earlier, it turns out that the lower vacuum hose from the Bypass Valve was completely blocked off by being "connected" to this here(1st pic). And this threw me: P0455, P0457 & the "Check Fuel Cap" light. I unplugged the vacuum line to ambient...
I did some looking into my bypass valve and it turned out that my lower vacuum line was blocked off, when it is supposed to go to ambient pressure? I removed the cap and had no issues on a short test drive. I will have to drive some more to be sure, but it's looking good so far! Fingers crossed.
Where did this PDF go? Can someone send it to me?
https://www.svtperformance.com/forums/threads/03-04-cobra-vacuum-lines-documented.733504/#post-15593026
I have a strange issue with my 03 cobra.
About 50% of the time the boost only goes up to about half of what it's supposed to, and the car seems to sound more muffled when it acts up.
The only mods I have are; a pulley, intake & exhaust.
Changed sparkplugs / oilfilter / oil / cleaned airfilter...