1992 Chevy Corsica How can I fix the problem to make the sm

Help, I have a strange smell that has come back. When I crank the car, it smells like the antifreeze is cooking and the smell seems to get stronger with the windows rolled up. I replaced the heater core in the car about 4 months ago. $200.00 worth of repair work and parts. The shop said this would fix the problem-it did but- am I going to have to have this replaced yet again? What is causing the heater core to go bad? I bought the car second hand, the people said they hadn't had any problems with this but did notice the back floor board behind driver seat was wet at times.-No raining in or spilled liquids were ever the cause, they never understood why the wet floor. Please help, I've moved to a new area and am unsure about car repair. I've no one to ask to make sure I'm getting the right info. And cost for repair.