I can tell you for certain if your having problems with 0.5mb fixed speed then things would be much worse with a rate adaptive service. once you go over 60db attenution then rate adaptive can become tetchy.
On a fixed speed service with typical snr the limits are <43db then 2mb is ok, <60db then 1mb is ok. Given your snr for a fixed speed is 22.4db then this is an indication the line is good (general guide for fixed speed, 6db or less and you'll probably drop sync a lot. >9db is the target. the teens is average, 20's good, 30's very good)
Dispite your overall stats being good and the fact your still getting drop outs means you still have what appears to be an intermitent problem, although it would have been interesting to know what the actual attenuation loss is (if this was say >65db then you may be suffering from a long line issue. In which case definately stay away from rate adaptive otherwise you'll probably get a throttled back service running at 0.1-0.25 mb