I also have an RCA 61 DLP projection TV. I believe I purchased it back in Jan 2005. About a year or so ago I noticed the same thing and went into the RCA config menu. I turned the lamp seting to the lower level to serve life. I also turned off many of the enhanced features. The TV would still do the flickering but only every so often. However, about a week ago it started doing it all of the time. I have unplugged it and tried everything with no luck. It will work fine for a little while after it is first turned on, but after awhile it just starts the flicker and won't stop.
Last fall I bought a Samsung 40 LCD TV from the same store I bought the RCA from. I mentioned that to the sales guy and he stated that they had so many of those things returned. Not all RCA TV's in general mind you just thier DLP units they sold.
I don't think it is the lamp they either work or they don't.
I've found a few links that list many folks with issues. Lots of folks have replace many parts and still not fixed there unit.