review when last tuned. it may be due. use premium type spark plugs(e3) as replacement. may want to spray WD-40 in to old spark plug threading prior to extraction and let set for a couple of hours. these engines use a low thread count spark plug which may make them brittle and prone to breakage. a lot of mechanics won't even touch them because of this, however this method works well. be sure to coat threading for new plugs with never sieze. examine ignition cables for sign of wear or damage, and replace if needed. rec. performance grade ignition cables for replacement(better conductivity). accel's are good and are more cost effective than OEM replacement.
rec. fuel system clean. ethanol treatment to 1 tank premium fuel. ethanol blend in fuel causes build up fuel system, combustion chambers, exhaust system and causes increased wear to spark plugs.
rec. oil stabilizer with oil change. this will recondition seals and cylinders(improve compression).
rec. upgrading conventional air filter to high air flow design(K&N).
these are things you can do on your own.
you really should get diagnostic to find any other issues you should be aware of.