Are any of the antennas for your main wireless links facing west into the setting sun?
An old time microwave tech I used to know told a "war story" about troubleshooting a corporate link that had weird problems every morning about the same time of day. It turned out that the antenna on the customers building was pointing dead into the rising sun and the noise swamped the link.
In addition to light, the sun produces lots and lots of RF. Satellite TV and telecom systems regularly have problems with this. Monitoring the S/N ratio and error rates across the time period should tell the tale.
The other obvious guesses would be volume of traffic (most places see an evening peak in network traffic) and other sources of interference. Got any friends in the local mobile phone industry? Ask them what their peak usage time is.