From your brief diagnoses stated, it sounds like a bad Video Cable.
The Video Cable routes from the motherboard, across one edge of the Fan Assembly, and under the left Hinge assembly,http://www.irisvista.com/tech/laptops/Toshiba-Satellite-Pro-L450-L455/big-body/laptop-disassembly-19.jpg
With just the usual opening, and closing of the laptop, the Video Cable's sheath may be damaged. This means the tiny wires inside the sheath may be damaged.
Or it may be the screen Inverter that is damaged, or going out,http://mcccomputers.com/shop/lcd-inverters/3666-new-toshiba-satellite-l455-s5009-l455-s5045-lcd-inverter-pk070006t20-k000087330.html
Or the LCD screen assembly.
Could be the graphics chipset.
1) Laptop OFF, monitor OFF, connect an external VGA monitor to the laptop's VGA connector, on the back of the laptop.
[ RBG (Monitor) port ]
You can use a CRT monitor (Looks like a small TV), or a flat LCD screen monitor if it has a VGA cable, for the external VGA monitor.
[ Example of a VGA cable, and a VGA port (Connector),http://en.wikipedia.org/wiki/VGA_connector
Turn the external VGA monitor on. If a CRT type allow it to warm up.
Turn the laptop on.
By default the external monitor should show the display, also.
1) Press, and hold down on the Fn key, and simultaneously tap on the F5 key. (The F5 key is the Display toggle-over key)
Not showing on the external VGA monitor? Perform the above procedure again.
There are several display options.
External VGA monitor also show the same symptoms?
Bad graphics chipset. Replacing the motherboard is the most cost effective method, and easiest method.
External VGA monitor does Not show these symptoms?
Problem is the Video Cable, or screen Inverter, or LCD screen.
Due to movement of the LCD screen, which means movement of the Video Cable, my money is on a bad Video Cable.
(A Video Cable can be unplugged partially, real easy, on the back of the LCD screen.
It MAY look plugged in tightly, but may not be.
It can also be unplugged real easy when assembling the Display Assembly, to reinstall back on the laptop )
Check the Video Cable's wires with a multimeter. Perform a continuity check of all wires in the cable. (Multimeter set to Ohms)
Have an assistant also gently wiggle the cable, as you are testing. See if there is an intermittent break, due to a stretched/broken wire.
Or just buy a replacement Video Cable, and use it.
For additional questions please post in a Comment.