My cable modem has been pushing the limits of the allowable upstream power levels (getting between 55-58 dBmV). This is causing the modem to reboot. Is there something I can do to reduce the upstream power level? I'm not entirely sure what causes this - could the number of things plugged into that surge protector affect it? Can using a different outlet affect it?
This is almost surely due to a wiring problem. The most sensible thing for you to do is to call the cable company to explain that your DOCSIS connection is unreliable and get them to send someone out to troubleshoot it and fix it. If they give you the run-around, mention that you're planning to switch to a competitor of theirs, and see if that helps.
If your downstream levels are decent (above -10dBmV or so), this could be due to a loose connection. Loose connections can sometimes impair low-frequency carriers without impairing higher frequencies very much. Tighten every F-Connector you see.
Inability to communicate upstream shouldn't cause a cable modem to reboot, but it WILL eventually cause it to re-start the sign-on process from scratch, which can take quite some time. When this happens, the blinking pattern of the cable modem's lights will look fairly similar to a fresh boot.