I am pretty sure its a warped bed - I have the glass now too so will be trying with corner blocks

I also came to the conclusion that it is worse with the windows in my office open - the small breezes that pass through the printer seem to make the abs lift and curl - so when printing in ABS i cover one side of the vents and have printed a louvered vented panel for the other side to cut down and direct the air flowing into the machine away from the surface of the print.

I swap back when doing PLA and cut down on the temperature but I never have much luck... it may be getting old though as its been in the printer since i got it about 2 months ago