Hello Lisamrein
Of course, it could be a defect. But when you write "most of the time", it means that it still works from time to time. Clean the sensor above the viewfinder (a 10 x 4 mm slit), which switches off the large monitor when you look through it. Use a damp cotton swab. The slit is very narrow on the A7. You may have to remove a little cotton wool from the cotton swab first. Only damp - not wet. You can also use 70% alcohol - but really only damp.

Register immediately as a warranty case.

Does the display in the viewfinder still work? If necessary, try to load the latest firmware.