I have had my Redmax EBZ8001 for over 10 years now, LOVE it! This year is has started a new problem. Starts up fine, but when for about the 15 to 20 minutes it will not get to full speed. It will only get up to about half speed, but then all of a sudden it starts working fine, almost like it needs to warm up. Tried a replacement carb, no change so I put the original one back on. New spark plug, no change. The only thing I have noticed is that there seems to be some oil build up at the bottom of the intake (area after I take the round intake tube from the air cleaner to carb off). Used to run better when I cleaned that with carb cleaner, but now that is no help. Per the user manual, my gas mix is 2.6 oz to 1, using premium gas for the mix. Had the pistons replaced about 5 years ago because I was using regular gas. Any ideas on what the issue can be?