Have you maybe enabled automixture in realism settings ? settings per manual ? fsx sp2 or acceleration installed ? any weather addons ? accusim with all patches installed ?
Well, estimated absolute ceiling for the Cub is more or less 14,000 feet, will need to check this out ,
these figures, you provided would suit more to a Super Cub, strange the engine even worked.
Edit: Checked, with full tank, no Heidi, excellent engine, starting at Lukla, i could crawl to 11,500 feet or so, then stalled, i guess fate wanted that it was close to a mountain wall
, check your settings carefully, if there still will be an issue, i bet someone with more fsx knowladge will come by