Last year the heat went in my car. It didn't completely go, but it was barely kicking out any heat at all. I had the system flushed, and it lasted a few weeks, but then the heat went again. I then had it flushed again, and still it only lasted a few weeks. I was told that too many flushes could cause damage to my engine, so I don't wanna do it many more times if possible. Fortunately, the really cold weather was over by then, so I wasn't too worried about getting it fixed again, because i had luke warm heat still. Now that the weather is getting colder, the heat is not there again. What is the problem? A mechanic told me that the heater core is most likely shot and will need to be replaced. I don't wanna shell out that kind of cash if it's not the problem. My rad is topped off with Dexcool, so I know that's not the problem.

I have changed the tstat in it, and drained all the air out of the system.

Any ideas if it could be anything else?