plasma TVs have a lot of circuit board that drive the screen, one of them is causing the issue. Depending on your skill and tools, we can proceed several different ways.
Repair center = they will charge you probably $200 - 400 to fix after a part is replaced, depends on if it needs a board or a component
you repair/diagnose = many of samsung flat tvs have bad capacitors (caps), the caps will bulge (*see pic below) if they are bad. you can open and look for these. they are solder in parts but can be easily replaced by a savvy person with a little soldering skill.
if no caps are bad :
if you have a voltmeter, we can check voltages when the picture drops out and figure out which board is bad, or where a missing voltage may be.
if you give more info we may be able to narrow it down somewhat without opening up the tv set. If caps are bad, the problem will happen more when its cold, and go away after it warms. if a semiconductor (chip, transistor, etc) is bad, the problem will usually happen more when its hot, and work good when its cold. you can also try kind of bumping it around (be gentle) and maybe something is loose like a cracked solder connection or cable. any extra details about when the pic goes out can help too. do you see a glow or anything when its out? do you see it get sparkly or pixelated before it goes out? any clues can help..
let me know how I can help, we can solve this.
bulged cap - good ones are flat on top: