Try a good tune up before anything else...plugs, wires, O2 sensor, fuel filter, air filter. It seems like your first problem was a vaccum leak. Start simple, then worry about major components. I doubt your problem is with the EGR valve unless it is failing to seat all the way and is allowing exhaust gas to leak into the engine at all times.

Unless you've just had major oil control problems, you probably won't find much buildup in the intake of an FI engine. Since they don't have fuel running through their entire length, heavy carbon deposits usually aren't a concern.

Pull those spark plugs and tell us how they look. I'm almost wiling to bet your problem is related to a basic tune up item.

 

Seems like you have checked out some possible causes. How's your heater out put? Check out your thermostat if its opening incorrectly or stucking it will effect gas mileage.