200 miles a week is a 20 mile commute, this is fine mileage. or are you making a bunch of 1 mile trips like doing sales rounds or something? Its not the number of miles a week you drive, its how far you drive each time you start the car that is important.
Short triping is terrible for cars, take it out atleast once a week for a good 30 or 45 minute drive on the highway, go as fast as you dare for atleast part of it. this will cook all the water out of the crankcase and the acid out of the exhaust.
What kind of fouling? if you have leaky valvestems, you may as well buy the cheapy Bosch plugs and just keep tossing them in. Switching to a more expensive plug will accomplish nothing if you are having mechanical issues.
If you need to modify your driving habits, do that first and see what results you have, then think about different plugs. Going to a hotter plug might be benificial. I would use NGK over Bosch if available. Avoid the cheapo Bosch platinums, they suck.