Hmm, replace fuel and air filter (Which you stay on top of anyway) which is good!
Hmm injectors, sometimes the injectors themselves can get dirty, ever run the fuel tank right down (although fuel filter deals with this normally).
How about a redex injection treatment??
It could be an issue with MAF being dirty, might be worth researching on the cleaning/replacement of that.
Have you tried taking your battery out for 10 minutes, i believe this completely resets ECU values (Stored over time), then you need to drive your car for the next 300 miles steadily so the car records values of how the car is driven (Instead of having fixed values i think marea learns? correct me if this is wrong!!)
Any heavy exhaust fumes?
Cambelt skip a tooth? timings out?
I'm brainstorming mate, i can't give you anything accurate, but you could slowly DIY work down the issues i've listed, but more members will continue to help!
Is there anything that has happened before the car started running sluggish??