V3SE losing height on Z while printing

My V3SE has all been fine, day in day out, no problems with first layer. Today something odd is happening whilst printing the layer the Z is dropping down, having to raise the Z offset by 0.2mm at a time, done this for 1.36mm so far and about 2/3rd through the layer and the head has dropped again, z-offset back up another 0.2mm etc. Checked screws and belts, gantry angle, gantry level etc. but still keeps on dropping.

That’s strange.
Did anything change like something new you added…?

Can you move the print head up and down to see if there is some play in it…? I’m guessing there shouldn’t be any…

Z motor getting extra hot…?

MIcroswiss flowtech which can’t be anymore than a couple of milligrams different, was fine last week with it. Stepper temperature is fine. Just in the middle of factory reset just in case.

Seems to have started with installing CP5.1, gone back to the old version but still not back to normal.

Removed all version of CP higher than 5, including all of the config files. Seems to have reset my 4.3.8 also, at present printing normally again but those prints will be in the bin as there aren’t the number of walls I usually print and the infill is wrong too. Love to know what was causing the height problems. Will see if I can break it again with CP5.1 in the morning.