Shower sometimes not hot

I am having an issue with my shower. Every so often the water is not hot. It is luke warm and then goes cold after running it a while. When this happens the water in the rest of my house is also luke warm. After running the luke warm water until it goes cold and waiting for a while the water heats up to the normal temperature. I have an electric water heater that was just replaced last year. The problem seems random. Sometimes I will go a month or month and a half with it not happening and then sometimes it will happen up to three times a week. Any help or insight would be greatly appreciated!

Intermittent things are a pain to diagnose, but here's a couple of things you might check: some single-handle valves, when they get worn, can allow a cross-over from hot to cold, and this can cause mixing - this can be random, depending on where the handle is left; the (probably) upper element or thermostat on the WH might be defective, or the leads are loose. There's probably a lot more things, but this might get you started.