scooby-wrx

I've had a bit of trouble with my car, it looses almost all power and mis-fires when I try accelerate. I had a mis-fire a while ago and Subaru replaced my plugs, fuel filter & pump, MAF sensor and it worked for about two weeks. One of the mechanics is sure its because of the unleaded I'm using, but I've tried Shell, Engen, Caltex and BP and I still have the problem. When it works (sometimes) there's a slight humming noise coming from the engine when I boost.

Sometimes it runs fine, then it will just loose power and cough out huge clouds of smoke. Here's a pic of how white my exhaust is burning.


Does anyone know what the problem could be? It's a 97 JDM WRX only mod is the exhaust. Still have a cats.

Thanks,
M.