Brakes feel soft sometimes but firm other times, anyone dealt with this?

I have a weird issue with my car that has been driving me crazy for weeks. Most of the time the brakes feel normal and stop fine, but every once in a while the pedal feels soft and spongy like it sinks closer to the floor. It is not consistent so every time I take it to a shop they cannot replicate the problem. I have been reading about vehicle stopping system troubleshooting and learned that intermittent issues like this can be caused by anything from a failing master cylinder to air in the lines or even a small leak that only happens under certain conditions. A mechanic friend suggested bleeding the brakes even if there is no air because old fluid can cause issues too. I am worried it might be something more serious like the ABS module acting up. Has anyone had a similar intermittent brake problem and actually figured out what caused it?