Connecting to TV -screen blanks and seems slower

I have a new iMac that is connected to my TV for occasional streaming of web content. I use a tunderbolt to DVI and then about 15' of DVI to DVI to connect to the TV. Now my TV is 10 years old -it is a 1080i CRT and I don't know if that's the problem.
But everytime we power on or off, the Mac goes blank for about 3 secs. And I think it runs slower when that is on -even we we are not viewing the TV through that input and we are watching a DVD or something. I haven;t tested my the slower theory -it runs really slow sometimes and I haven't conclusively tested to see if it is 100% linked to the TV being on and what unplugging the cable would do. I wanted to see if anyone had any thoughts on this first.