2001 Mazda 626 Running rough and no power!

Hi
I have a problem with my Mazda capella s/w (imported from Japan to NZ 3 years ago).

A friend says that it is faillng to fire on all four cylinders at times, ie. The problem is intermittent. When this happens it runs rough, smells like its burning petrol or something, and I can't get any power even with accellerator flat to the floor. Then it will come right.

Have taken into garage and auto electrician 3 times. They reset the fault codes and have checked since problem occurred again, and there are no new codes (there has not been any dashboard lights assoc with this problem). As the problem is intermittent they can't figure it out but have checked the plugs, leads and replaced the fuel filter.

Can anyone tell me is this a problem specifically with this model and is so, what is it? Alternately, any ideas what I can do as i'm at wits end, this has been giong for months now and I can't use or sell it in this condition.

thanks!