Yes, the plugs would be the obvious place to start. They're pretty expensive new (around £80 for the set - look on fleabay) so often get neglected, or cheaper non-spec' plugs used instead.
Take yours out and if they look shagged out, replace them.
Use the opportunity to remove any oil that's accummulated in the plug wells, either from spillage while topping up ...or a leaky rocker cover gasket - and check the condition of the HT boots where the lead connects to the sparkplug and also to the coil. Clean everything and if anything looks dry, brittle or corroded, replace it.
The MAF or other sensors could be an issue too... but if you service the beast (you almost may as well while you have the plugs out) then fit a standard paper filter rather than a panel doodah. You want to get the beast running properly before you introduce a non-standard part. When you have the air filter off, clean up the throttle body and geeeeeently wipe around the MAF sensor (it's like a little metal blob sticking up on the end of a piece of fuse-wire, so don't ran a huge cloth down there.. you'll break it).
Replace the fuel filter too.. it's probably the original 12 year old one that it left the factory with. It lives under the car, under a plastic cover, where the right hand rear passenger would park their arris.
Then fill the tank up and add about half a pint (250ml) of paraffin to the petrol. You can buy paraffin from Homebase and B&Q - it's called "Turpentine Substitute" (don't use White Spirit) and it's the main constituent of the proprietary fuel system cleaners you can buy. Add 250ml per tank until the paraffin is all used up...
All this should solve any "build up of gum" issues and make sure that there's no impediments to everything working okay.