Help: Strobing on rear projection TV

I purchased a used Sony 42 inch rear projection TV (it's about 4.5 years old) and there's a slight but noticeable strobing or flickering when playing DVDs. (I haven't been able to check out the regular tv image yet because I don't have cable hooked up.)

Is this normal? Is it something that can be fixed? It's very annoying and I don't expect an honest answer from the seller.

Thanks in advance.
You know the bulbs dont last forever so after that long I wouldnt be shocked if its replacement time. I know my front Projector will flicker a bit when I need to clean filter...always around every 100hrs. I know you have rear but all the same it could be a bulb and just maybe its dirty inside......infact after 4 years its likely.
every 100 hours! jeez.

Thanks for the tip. I guess i'll try to get a manual for the tv and figure out how to change the bulb. do you think it's something i'll have to get a tech in for or is changing the bulb relatively user friendly?
so you are saying in 4.5 years you only used it 100hrs? No mention of hours were in your post and I may be wrong but thats one idea I had.
chad, i was saying i'm surprised you clean the filter of your tv every 100hrs. i bought my tv used and have no idea how many hours are on it but now i have a worse problem...there's a hum! it's worse with the dvd player than the satellite dish but it's there either way.
Try to lift ground on DVD or check your cable ground or even an inline filter from cable company may cure it. Sorry your having sucks.