An easy way to test if it is the heater core is to get one of those T's for the radiator flushes and take the 2 heater lines as they go into teh firewall and connect them together using that T.. (it doesn't have to be a T but the T's are easier to find than a coupler)....
Now that you have bypassed teh heater core, drive around.. If the problem goes away you know its the heater core..
Changing the heater core ususally isn't hard (I haven't done it on a 2nd gen though).. Normally it is just the 2 hoses a few screws/clips to get to the heater core and then you just swap it.. not hard once you figure out where all the clips/screws are to get to the heater core.
~Mark