I have a 12 year old firplace that starts fine with the pilot but goes out after 15 or so minutes... as soon as it gets hot. I went on line and most of the advice was the same... to change the thermopile. I changed this and have the same problem. Now sometimes the burner won't turn on the same as it did before. Any suggestions?

Check to see if there is a temperature safety control. This will cause that kind of problem.