Well you most likely have a bad lamp; I say this with about 95% certainty. It is very common for a lamp that is getting weak to get darker and darker...
So what I would recommend you do is replace the lamp at this point.
Do you know how to do this by any chance? If you still have your owners manual there will be instructions in there on how to replace the lamp unit. If you do not have this I will assist you in getting it out.
Also you can purchase a new lamp many places online, I recommend http://www.dlplampsource.com/ they are a good company and get the lamp to you quick. Also they will sell the lamp in two different ways, the first is just the bulb and the second is the bulb in a housing. If you get the whole thing in the housing (costs a little more) the job is a snap. If you get just the bulb then you will need to disassemble your housing, take out the old bulb and install the new one.
Let me know if you need more assistance from me, I will be happy to help.
If you would like any additional information or have more questions please don't hesitate to ask!
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