Is this happening to more than one SM? I would start by working from the lan directly connected to the AP and SM on the other side of the router. From the router can you ping the SM when this is happening? When it is happening check the arp tables of the router, AP and SM to see if they have entries from each other. It sounds a little like an ARP entry time out but for some reason not getting readded by an ARP request. Looking at the ARP tables may give a clue.
Yes, it's happened more than one SM, about 3-4 SM. When it's happened, i tried ping from router - AP is pinging, SM no, i saw ARP table in router - AP and SM was there. i thought about problem with arp on router and reseted arp on router and again tried pinging - AP is OK, SM not pinging. Again saw ARP - AP and SM was there again, MAC is true above them. I saw arp on AP - SM and router was there. When i make telnet session and enter on SM from AP, ping beginning and ARP on SM has been ok. Once - i go to SM from AP over telnet and saw ARP (ping not beginnig), router NOT was there, i to do ping from SM to router and ping started and router added in ARP table on SM. I think this is arp problem too, and i think this problem on SM or AP side, but i don't know how debug this...
Hmm we're still on 11.2 for our PMP-100 gear (13.2 on PMP-450), so I am not sure if anything really changed, but...
It's been a very long time so I can't remember all the specifics, but I do recall having SOME kind of issue similar to this. These are the settings I've used for the past few years: