Thank you for your quick response. My speed is great at any times except for this one spike (speedtest.net is at 8.6Mbps with 23ms ping at a normal time). It's been a while since I tried an isolation test, so I'll try that again. Because it only occurs for 15mins, it'll take me a few days to perform it since I have to wait for 10pm each night. I'll investigate the house to see if there's anything that may be pinging something at 10 that I've overlooked.
I appreciate your help!