Is this a "DLP" (Digital Light Processing) type tv? If it is, sounds as if the "light engine" is going out. Unfortunately if that is the problem and you don't have any type of service contract on the tv, will probably be cheaper to buy a new tv than to attempt to get fixed.
Going from experience here... had a Hitachi 50" DLP and it started to develop what folks on the web called the "Hitachi blob".. It was basically a red cast to the screen that started out small and hard to see, then got bigger and bigger and redder and redder. Researched it and found the cure was to replace the "light engine" (at a cost of about $650 USD a little over 3 years ago)... Initally we decided to try living with it and then we got an offer from the company we bought it from (it was already out of warantee) to buy an extended service contract. So we jumped.. bought the contract, waited a couple more months and called the company.. They sent out a tech, diagnosed the light engine, ordered a new one and replaced at no charge to us other than what we paid for the contract (approx $10 month, so about $30 at the time).. It was worth it and the tv was like brand-new again... but we couldn't have done it if it was all out of our pocket.
Again, don't know if this is what you've got, but just an fyi...