I have a 25cc strimmer that starts OK and will run, then dies?

Any suggestions please as to what the problem is. It will run for 10-15 minutes then die slowly,that is it doesn't cut out.Pleanty of fresh fuel with correct amout of oil. Then it will run for abour 5 minutes then die. Try it the next day it will start OK and run for 10-15minutes and it will carry on as above. Still behaves in the same manner if the plug has been changed.