I would take it to a reputable tuner

Also sounds like you have the wrong guage fitted. Vacuum guages are for normally aspirated cars, and it is working correctly if when you come on boost it reads "poor" (Showing positive pressure in the manifold (Boost)) and when you back off it reads "decelerate".
Best thing to do is throw this guage in the bin and replace it with a boost guage

I doubt the poor running of the engine has anything to do with the wrong guage been fitted, although obviously someone who doesnt know what they are doing has fitted it, so it will be worth checking the connections...

Hope this helps a bit!