There are a coupla "known" defects in the PowerBook line. One is that the cable that attaches the LCD to the video electronics can work loose over time. Call 1-800-APPL-SOS (I think.. the number is in the manual) and expain your problem. Have your serial number handy. They will wisk away your baby and install a screen cable 'shim' (somthing to hold the cable in place) and do any other standard fixes, and have it back to you in no time (2-4 days total).
My 140 flickered until it blanked-out, they fixed that and installed the floppy shield (140-170 only) at the same time. Since then, no problems!