Issue with a B&S 17.5 Intel HP Engine

I have a lawn tractor that is 15 years old. The engine is giving me a hard time. After about 15 minutes of running, it starts to bog down like it is going to stall and then is fine. The about 10-15 minutes after it does the same thing. Because of the age I replaced the carb. I also drained the fuel take and put in fresh gas and a new filter. It is still doing this. Anyone have any idea what I should do?
Jeff