Did you set the tension properly on the belts you replaced? Too tight, and the engine will shutter. Too loose, and it'll squeal. My guess (especially since it started after you replaced the belts) is that you didn't set the tension right. Gaskets are an easy fix. Probably took the valve cover off before, but didn't replace the gasket like you were supposed to. Engine mount, also easy.
When was the last time you changed the plugs, wires, cap, and rotor? Changing those will help your gas mileage. I hope you changed the timing belt when you changed the other belts. If not 90K is when your supposed to, and you'll have to remove the other 2 again to change it.

My advice is to get a copy of the dealers scheduled maintenance at each interval, and follow it starting with the 90K mile service.

I don't know the car, but I don't think it'll take much to make her feel like new again.