Coolant Issues

Recently I have noticed that the temp of my car has been going up to 260-270 when idling or city driving, it will go down to 200 when on the highway. I noticed over the last month that the coolant in radiator keeps going low (which I am guessing why the temp is going up). I tried runing the car and checking underneath and pressure test but no luck. I just rebuilt the top end of the engine about 7 months ago (so new gaskets on top) but left stock cam in, put new radiator in about a year ago with new hoses and clamps, so I am guessing it is either the water pump gasket or timing cover gasket. I figured since I will be taking the pump off just going ahead and replace it. I wanting to get some opinions on some. Also if the may be a better way to test for a leak so I can save myself some time.

Thanks for the help.

+1 on the heater core but I replaced that last January. Looks like a fun afternoon ahead for me. Good idea to change the chain while I am in there, about 70K on the motor or should it be good for awhile?

Any brand of timing chain better than the other, I figure I will bit the bullet and change it. My oil pan gasket is leaking a little and it has been at least 5 years, according to the guy I bought it from, since they were changed.

I was also debating about getting an electric water pump. I was wondering can you run the pump with the OEM alt. and can you run it without getting an electric fan.