BJ is right on the head gasket,dont worry about it.as for the coolant leavel conatantly going down mine does it also and have yet to find a leak.just have your freind keep an eye out for it. you can also place a piece od cardboard under the engine at nights to see where and if any antifreeze is dripping,as well as pressure test it.
as for the chugging and such test the o2 sensor,my jeep did the same a couple years ago.
here is a list of things that should be checked and cleaned,repaced or repaired as needed.

iac-idle air control should be removed and cleaned from carbon and other deposites.
throttle body should also be removed and cleaned.
check the fuel pump ballast resistor that is located on the left front fender towards the front.it is an off white block with 1 screw holding it in and 2 wires hooked to it.remove it and look at the back you should see a rope about 1/4 in dia with wire around it.make sure it isnt loose or damaged.heck it is only a few dollars i would get one.espeacially if you see the rope.the new ones are sealed in the back.
check the coil to see if it is putting out enough spark.or as BJ says replace it every 20k or so.
i would also run an oil system cleaner through the engine.gunk makes one that comes in a silver can like the old 1 quart oil cans that works really well.
i mentioned cleaning the throttle body and iac so i would go ahead and do a fuel injection service as well.what that boils down to is an o2 sensor safe carb cleaner,an additive for the fuel tank and if you can find them they make a can of cleaner that hooks to the fuel rail wher the fuel hose goes in to clean the injectors,valves and combustion chamber.
i would really start with the o2 sensor though with the rough running,and extra deposites on the plugs points to an o22 sensor that was slowly failing now compleatly gone.