Jason,
I've been running the same combo as you and had the same problem with the valve stem. I finely just tightened down the stem nuts and never had a problem since for the last 1000 mi.
The 803 has a stiffer side wall than the IRC and Michilin which leads to the lower air pressure and more tube creep. When my 803 dies I will switch back to the IRC
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07 GG EC 300
05 KTM 250 exc
99 KTM 640 enduro
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