I'm not a huge fan of the california yeasts, but that's just me. I've never tried American wheat though. I'm thinking your problem is yeast related though since you talked about gas. Anytime you drink the yeast, that's what'll happen. With wheat yeasts, a lot of that yeast stays suspended in the beer.
You say you fermented for 6 weeeks...was that six weeks in primary, or split up? If you left it in primary for that long, you may be showing some early signs of autolysis. Some additional aging may help.
On your next few beers, try a neutral yeast strain like s-04. Also, only use primary for 3-4 weeks. This will determine whether it's the yeast causing bad flavors or whether you have deeper issues like water chemistry.