Could just be water on the plugs check and see if there is any down the spark plug chambers. The first thing I would do, as Jay said is to get a boost gauge fitted, its one of the most important things to have in a turbo'd car. You could also get the fuelling checked as it is now on a rolling road, a power run normally costs about 50-60 quid and will let you know how the car's performing and give you a base figure for improving on with any mods you do, plus it will give you peace of mind that all is ok now.