Machine digging deeper

[BruceR]

Hi, Could anyone please give me a bit of guidance. My CNC is becoming unpredictable.Yesterday it stopped, saying there was an external stop pressed when there was no-one near it or even in the workshop. Then I sent it to home and tried to home it but it just slowly crept down towards the spoilboard. I had to stop it. I tried several times to home it but ended up turning it off. It was also cutting deeper with different tool paths when they should have been the same. I stopped using the z sensor device and started using a piece of paper to set z,
Today - completely different file. I set the Z using a piece of paper. Half way through a sign it started cutting much deeper. It had already cut a good bit perfectly. It also didn’t lift properly between cuts and scraped across the wood.
The z flow is regular without any hiccups and it moves in every direction smoothly. The machine looks and feels as solid as a rock. I suspect software but I don’t know what.
Can anyone please tell me what I should be looking for.
Thanks.