Ender 3 Z-limit now unreliable - possible causes and solutions? I had a couple of recent nozzle/bed crashes, so I now frequently do a manual bed levelling. I do these while the bed is heated to allow for expansion. 

Today i found, after levelling, a subsequent print could vary from having too much clearance (paper moves very freely) to less than no clearance (which left deep grooves in the bed as per the photo). 



What are the possible causes? Is it just a dodgy Z-limit switch or something else? What solutions are available and at what cost? Obviously an ABL seems essential now, but requires a lot of hardware and setup. 

Update

Just happened again so here are some more details. 

Printing PLA this time. 
 - Preheated bed. 
 - Levelled bed all over and got a very good raft. 
 - Print failed later so aborted. 
 - Restarted the same print. 
 - Nozzle gouged out a uniform depth trench around print perimeter. 

The uniform depth of the gouge indicates a problem related to Z-axis only as bed remains flat and level, but height is out.
I've just bought an Ender 3 Pro and as a guess I would look at the Z axis stepper motor to see if it is not holding position once it stops... the stepper motor has power on it all the time it is stopped to hold it in position.

I would check the plug that feeds power to see if it is damaged or not making good contact.