My car is actually a Mazda Lantis, which is similar to the Protege. The problem is very sporadic and has been occuring for just over a year. One minute I'm driving and the engine is fine, then next it starts shaking, sounds very rough and loses power and/or is extremely slow to accelerate. In fact you have to depress the accelerator further down than normal in order to get it to move and it's even worse going up hill; it barely moves. There's a strong gas smell and I've been told it's noticeable to vehicles behind me as well.
As I mentioned, it's very sporadic. When it's misfiring, it can last 5 seconds, 5 minutes or even for the entire drive. The misfiring can go in and out and there doesn't seem to be any particular cause; it happens when it's idling in traffic, going up a hill, driving on flat road. Some times I get out of the car to go to the grocery store or drop off my children, then get back in minutes later and it misfires on starting. Weeks can go by without any problems where the engine is running nicely and then it happens again. Or, as it usually the case, it occurs at such dangerous times as just before you turn into incoming traffic. I've learnt not to take any chances when turning across vehicles or using roundabouts as you never know when it will start acting up.
We've had two diagnostic scans which didn't reveal any codes and I've been informed it could be anything from ignition coils to sensors. I'm extremely frustrated and want to know how to realistically diagnose the problem without randomly (and expensively) changing parts. Thank you for any help you can provide me with.