Here are photos showing the improvements after moving SM1 from its preferred AP to the adjacent AP. SM1 lost around 2-4db in signal and has lower peak rates but they are much more stable.
SM1 after move (yellow line)
Here is SM2 and SM3 both showing their rate stability went back to normal after moving SM1 off the AP.
Very odd. Almost makes me think some type of timing or scaling issue with the AP scheduler. I will try to report back next week once I have had a chance to try 5.10RC33 and move SM1 back to its preferred AP.