L32 after about 20 min. of use the picture flickers and finally goes black and it doesn't work again until about an 1/2 hour of rest. 20 min later its black screen again - (this happens on every input!!!) what is happening and what can i do to fix it?

There is something overheating in your tv...
the causes of this might be the vents are being blocked, maybe they have too much dust or there is a tshirt or a towel on your TV.

there could be some resistance, fuse, or transistor overheating making it to shut down to prevent a massive burn out briking your TV

if its a PC.. in wich im not sure if its a tv or a PC.. but if its a PC, its that the CPU isnt getting enough vent.. so replacing the thermal past from the CPU and heatsink will make the temperature go down, also, try checking if the small ventilators are working correctly.. if the dont, change them all.

to make cheap fast repair.. get an air conditioner or a huge ventilator and make the thing get some air