This thing runs great indefinitely as long as I don’t let it rest too long. But when I shut it down for say 10 or 20 minutes and start it again, it runs very poorly. Spits and sputters and just won’t get to smooth high rpms.
Next time I start it from dead cold it’s fine again. Same issue repeats if it rests a while.
Anyone know anything about this issue?

I wonder if vapor lock is an issue. Have you tried idling it for a few minutes before shutting it off?

Have not tried that but will next time. Yesterday when it was acting up I emptied the tank, blew through the lines to ensure no restrictions, refilled tank, pumped 10 times. Started right up but still had same issue. Always starts easily and idles fine in this condition. Just seems starved when I open it up.

This just got stranger. I have two of this model, one at each of my properties in different cities. So the idling before shut down did not help. Brought one from the other place this weekend so I could swap carbs before jumping to conclusions and buying a carb or kit. Had the same problem so I started using the other one and found that it does the same thing. Then realized that I had only recently changed the order of my work at this property and that’s when I noticed the trouble. Previously I had always done my trimming and blowing straight through at both places. Since I started alternating between mowing and trimming at this place I’ve had the problem. Wondering if it’s an inherent defect or if there might be something that fails after a couple years. These were purchased about 8 months apart.
One more thing, when they are running poorly, I can rotate it to the tank down position and it will run better for a while. But after a few minutes this fails as well. Doesn’t matter how full the tank is either. I’ve tried topping it off and checking the filter position to be sure it’s in the fuel. Also noticed that when this is happening the primer bulbs don’t work well at all.

Well you probably not going like this but these trimmer should never be operated other with the oil down and level. These engines don't like being operated in any other as they simply don't oil the cylinder. I have seen too many of this style simply ruin because lack of oiling. I hoping you haven't ruin it yet but I am thinking the ring is already sticking.
 
About the 4 trimmers that I know of that oils well is the Shindaiwa version for true separation of oil and fuel or the Stihl 4 mix system which is a true cycle; although, it uses 2 cycle mix.

The one that I just learned has the same operating trouble after sitting a while had never been operated in the wrong orientation so I’m thinking it’s unrelated to a sticking ring. Plus I’d think a sticking ring would affect performance start to finish. Guess I’ll just chalk it up to characteristics and do all my trimmer work straight through from here forwards until I wear them out. Thanks for the input!

I agree if you try to use the wicker as an edger the head is positioned out of level and can starve the unit from lubrication. So when edging I will rotate the shaft and handle 90 degrees to keep the head level.