When I attempt to make a print a thin, straight line of slight underexposure appears on along one side of the print. The frequency of this happening is unpredictable; sometimes there is no underexposed area, sometimes there is. I've checked the negatives and they are fine, so the problem must be in the enlarging process itself. I'm wondering if the problem could be my two and a quarter film holder with is old and a little sprung. I thought I check before buying a new one. Does anyone recognize this problem?

Thanks for your help, John N