its always a nightmare when this kinda thing happens.

first off - check under the hsn for any sticky fuel residue. thats the only thing my OS has had tuning issues with, especially after being left for a while.

what fuel are u using out of interest, what nitro and oil content? i ran byrons 25% race stuff thru mine for run in and after. it took a lot more tanks than i was expecting for it to run in fully, but that could be my technique, but... mine ran just as u described for quite a while when running in, was about a gallon of fuel before i got full power from it, and even then it was an accident... "i wonder if itll take any more leaning... bloody hell!!!".

uve probably tried most or all of the stuff below, but thats what i would try...

1 check the carb is spotless under screws etc.
2 check or replace fuel lines
3 swap the tank for a known good one if possible and try that
4 what plug do u have? try an OS#8 in there if u didnt already (should have been suggestion #1)
other than those regular checks... erm... is there another temp gun u can try? i never trust instruments or gauges much!

im guessing the compression is still good in the OS? i hope so!

my sh motor is sulking at the mo, so let us know how u get on please mate