The best thing to do would be to check the logs right after the problem starts.
The receiver is outside? Based on the Signal Level graph, it looks like the receiver temp started to get much warmer in the spring. (Weakest signal level decreases as receiver temp increases, aka gain decreases with receiver temp). Does the problem happen in the middle of winter?
Power draw increases with temp. So if you barely have enough power supplying the Pi, that could be the problem when temp is higher. Even starting or restarting a CPU intensive process could trigger a power issue if the Pi is already at it its limits.
You could set up RPI Monitor to check for power issues. That could help rule out such an issue at least. See: RPIMonitor
