Rob,

some thoughts

any airfilter debris sucked in the carb MAY still be there, and WILL cause a problem if they are, however the effect shouldnt be at full RPM, more so, it will not start or will idle rough.
Take a look in the throats, if debris is there, do a full pull-down and inspect. Assuming the engine ran fine last time you flew, expect it to do so again or look for changes SINCE then.

first theory,
sounds too lean. Removing the filters will do that, improving airflow to the motor. If it was run too hard when lean you may have internal damage, scored bores or broken rings from overheating in the upper cylinder area.
Does the RPM increase or decrease as you get higher?
The higher you go the less oxygen, meaning the engine runs richer. As you get high with a lean engine, it will rev faster, to a point. As you get high with a rich engine it progressively decreases RPM.

so,

#1, GET new fuel, dont use the stale stuff in the tank
#2 remove the new filters (you do have new filters yet??) and run up to full power on ground, check all settings including EGT if possible. Run for some time, there may be thick, old oil within the motor that needs blasting out.
There may be an issue with piston ring, main or big end bearing due to debris from the filter, also a clogged exhaust will do the same.
#3 put filters back and repeat process, note any changes.
#4 if there is a difference you have a filter issue, if not, an engine issue.

2 stroke engines never run quite as well as they did just before they seize, so too rich is the best option.

disclaimer: I know nothing, dont listen to me
Its is completely possible following the above will cause you to ditch head first into a line of school buses, killing hundreds and leaving you in a chair. Get expert help and sue them-
Blasted American law system.


bagger