Lots of On and Off can cause carbon build up in the exhaust, if it's not getting hot enough.
Used to have this with my 192t/ 193t. Make a big cut periodically get the exhaust plenty hot.
Took my screen out and muffler off to clean and replaced them. Lots of build up.
Sharpened my new chain to shorten the cutters.
Will watch for vibration bubbles. Thanks.
I had the spark arrestor screens clop up pretty bad when using Echo's Red Armor 2 cyc oil.
The screens were red in color.

After that I switched to their Gold stuff and hasn't been an issue so far.
Saw is too new to be plugged up yet.
Give us an update on what echo says. Any chance the fuel filter is in a bad position in the tank? Otherwise, I would fine tune the low and see if you can get the idle more consistent in various positions, i.e. on side, bar down, etc. I always check positions when tuning and I will say that my Stihl manual carb saws are easier to make consistent than the 2511. I have learned to tolerate it off the 2511 and keep the idle rpms up at 3200 [which is spec] which is well above stall speed. Just a side note - when my properly tuned 2511 does exhibit position dependent idle problems, it's usually after a cold or lukewarm start, or on one of those funky days like really high humidity or something. My Echo trimmer has similar idle behavior when cold. My Echo's use Walbro carbs, and my manual Stihls are all Zama I think.
It sure has been hot and humid as hell here the past few weeks.
I've only run about 4 tanks on the little guy so far.
Had a crap tune from the factory.
I have 5 echo saws, a trimmer, and their PB-580T blower in addition to my Still 193T and Husqvarna 390X.
Out of all my Echo gear only this 2511t and a 352T had crap tunes from the factory.
The CS-352 was the worst, got it running good and sold it.
It was a little under powered for my liking and and the relationship started off on bad terms so I ended it.
I did put a tach on the 2511t and got the idle up at around 3200-3300 between it's hunting spells.
It doesn't stall anymore which is good but it's doesn't seem to be at it's full strength yet at WOT.
After I talk to Echo I'll get in to tuning it some more and a few more tanks before modding it.