Hello, In addition to Randy’s thoughts, I’m wondering if you have a swing check valve in your recirc line. If so, it might be getting stuck open from sediment and then when you flush things it gets cleared out and behaves correctly. Next time the system is only giving you luke-warm water, try this: turn off the recirc pump, wait at least half an hour and then have someone run hot water. Feel the recirc line and see if it initially warms up. This would indicate water is running backwards from the heater and lets you know the check valve isn’t working.
I prefer spring check valves as they have a more positive seal.
Another test you can do is simply to turn off the recirc pump and close the valve in the recirc line that should be at the heater. If it takes longer to get hot water, but it’s as hot as you expect, you’ve isolated the problem to the recirc line. 😎