Second, you will not see much gains from a header of that style.
You can get good gains by porting the stock exhaust manifold or get a straight tube 4-1 header (good luck, I don't know of anyone that makes one, perhaps get one custom made)
If the tint has some metallic it might have some effect. I would think the mirror itself would have the most detrimental effect. My antenna is on the drivers side of the window up at the very top and I never have problems.
Mine does not have a return line. It should come off the opposite side of the fuel rail that the pressure line goes in.
DT, my car is an hour and a half away and every time I go to see it I get in trouble for now because of other things, so no... not so much go check it out.
First things to check in this case are, fuel filter and fuel pressure.
If you have pressure built up in the sytem the filter could be clogged and the pressure is holding the sediment in place, when you relieve the pressure it allows the sediment or whatever to flow back through the line and removes the clog.
I would still stick a pressure gauge on it and make sure you are getting correct pressure at the fuel rail, pumps can start to go out in warm weather and the sock could be clogging up. After that I would check the FPR, of course the pressure test should give you some hint as to if that is it or not.
Also pour in injector cleaners are great but remember the dissolved crap can only go one place, and that is through your fuel system. I would recommend paying the $70-100 to take it to an ASE certified shop and have them clean it for you. They will run a pressurized cleaner through it and keep it from staying in your system or going through your injectors.
before you go replacing things all willy nilly, be smart and test it.
When you pressed on the schrader to release the pressure, was your key on, or was it sitting idle. the sytem will hold pressure and then when you press that it will release it, if the fuel pump is not running you just let out all the pressure so it will do exactly what you described.
First, get a new gas cap, this eliminates this as a problem. I would check and make sure you have fuel pressure as you crank it. If you do, this removes fuel pump, pressure regulator and Fuel Pump relay as possibilities. If you have fuel pressure and everything is good at this point, post up with what pressure you have and any other anomolies you have noticed and we will go from there.
However if you do switch to synthetic do it in stages. Use normal, then a synthetic blend for a couple of oil changes, then full synthetic. If you switch directly over you can cause engine damage and will most likely have a lot of blow by. And remember, if you have leaks, synthetic will find it.