What is the environment of the area. Are there working SM's relatively close to the affected SM's? With the reported air delay I am wondering if possibly there is something in the way and possibly receiving the signal off reflections. How is the signal strength compared to other SM's about the same distance from the AP?
I was thinking further on this issue. Has the SM with the issue been replaced with another radio to see if the problem is still occuring? This may help seperate if it is a real RF issue or possibly hardware or software issue. Are you using a reflector or dish?