Make sure the CPEs are exactly aligned to each other while trying to connect them. Sometimes it needs several tries until the client finds the AP in a survey, but if the client is eventually locked to the AP, they usually sync faster. [Edit] Could probably be DFS, which prevents a client from seeing an AP if a radar has been detected on an auto-selected channel. See the knowledge base posting about DFS for a list of affected channels.

Oh, it's not that critical except on initial connect. A trick which worked for me, is to set the AP to fixed channel 100, reboot both devices and let the devices automatically re-sync/change channels if DFS demands it. Just make sure to lock the client to the BSSID of the AP. Also, the weather radar station often use the same channels, so there are usually enough free channels left in the 5 GHz band for stable links.

If anything 5Ghz P2P links are pretty stable despite being on pure-DFS bands ;)