FortiAP - strange behaviour, disconnect, freezes

[mcwz]

Hello everybody, I have a strange problem. FortiWifi 60D with 5.0.12 and FortiAP 221B with 5.0.7.

Everything works very smooth but there is one big problem. When I copy a large file (about 10 GB) from my notebook connected via wifi to my server connected via cable, after some time the AP "dies". Red light is flashing and it does not reconnect to the controller anymore - power cycle of the AP is required. I upgraded the AP to 5.0.9 and the dying problem has gone, nice. But now there is a new problem. Since I upgraded the AP to 5.0.9 I have annoying "freezes". When I'm connected to my server via remote desktop, about every 15-20 seconds the connection freezes for about 1-5 seconds. Feels like if the network has gone and then came back. Needless to say that working is impossible under such condition. Has anybody seen something similar or knows how to fix it? P.S. please don't tell me I should upgrade to 5.2/5.4 as I'll definitively don't upgrade to those versions. I don't need the features of 5.2/5.4 and I'm not a Fortinet beta tester ;-) Kind regards