Hmmmmm
1. you are using 5% nitro..... I use 15% nitro on my OS 40 LA
2. Your throttle opening should NOT completely close at low idle, try opening it up a ways and then start the process of tuning it down to an acceptable idle by gradually closing the throttle and adjusting the settings...... gradually aquiring a good idle....?
3. Bubbles in the lines arent just because of vibration, you could have a bad fitting somewhere in line, this would let air into your closed system, I think yoiu said you changed your tubing, if not, then by all means change it, inspect your fittings carefully for hairline cracks ect....
4. have you taken off the carb recently? you said you weren't sure if it was leaking air there, possibly a new o-ring or gasket of silicone or something would ensure that is not one of the many possibilitys which all add up to a problem................
I'm hoping all this info is more than just a jumble of confusion....... Cptn Crash...