I am looking for an answer to your problem, using google search, but so far I haven't found anything to describe your problem; what I have discovered is that this model of tv is very old & out of production now, & expensive to repair, if repairs are even possible - from what I can make out, for example, the main board is not available any more - so I have found a website where you can ask a Mitsubishi expert (I know it says mechanic, but there must be an expert that covers tv's, from what I read);
I am not quite sure what you meant by " the screen is sagging in the middle, top & bottom and intermitently becomes normal (no sagging) after a few seconds and will be normal the whole time it is opened."
If you meant that the picture bows (Curves) down at the top, & up at the bottom, then I would expect that it could be a problem with the projector bulbs;
My 42 inch Toshiba projector tv (now relocated to the bedroom, since I bought a 42 inch HD LCD tv for the lounge, with freeview built in) has to be opened up (With the power off & unplugged from the power outlet socket on the wall) every 3 months & dust off the bulbs, clean the mirror & the inside of the front plastic screen.
It is a nice tv, with a very good picture, but it is so old now, & technology has advanced so much now, from what I am reading, you may need to decide on what is best, either spend what may turn out to be a fairly considerable amount of money - something no-one wants to do in today's very fragile economic outlook - or, invest in a new tv, perhaps you may be able to pick up a reasonable LED LCD largescreen tv with inbuilt freeview tv channels & HD for a reasonable price, which will be equally as good as your Mitsubishi WS-65809?
I have also just found this link which may be helpful?
For projector lamps, try this site:-
Sorry I cannot be of more help, but hope this information has been helpful.
Please note that tv sets contain very high voltage components, & therefore I must advise you not to open the tv unit & attempt the repair yourself, unless qualified to do so, for your own safety; please take the set to a tv repair shop, or a person qualified to carry out the repair.