Sion

recommended spark plug replacement intervals vary widely from car to car and from one brand of plug to another. typically you should be able to get 50K miles on a set, but some will work fine up to 100k or more. if you dont know how many miles its been since they were replaced, it'd be a good idea to go ahead and change em. be sure to go with either OEM brand or a good name brand. I like Bosch double platinums, but they're a tad pricey.
-----Added 25/8/2008 at 01 : 44 : 37-----
two other things that could be causing your problems:

oxygen sensor gone or going bad...they're usually good for about 50k miles...if you are getting dirt mixed in with the intake air (due to the aforementioned hole) it could be fouling the O2 sensor (as well as the spark plugs, now that I think of it. slap a piece of duct tape over that hole). since the computer relies on the O2 sensor (among a few other things) to determine the correct air/fuel mixture, this could very well be the source of the problem. unless you are mechanically inclined, this one is best left to a professional

the catalytic converter. cats will slowly plug up over time, but once they get to the point where its starting to restrict the exhaust flow, it will cause performance to deteriorate rapidly. left unattended, it will soon cause the car to fail to start. have it checked by a reputable exhaust repair shop.

my bet is on the cat...vehicle is just old enough to expect that the OEM converter is at the very end of its service life.

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