If you are running through your Pioneer amp, I would try it running direct to your TV first to see if that works out ok, if that works, then it's your amp's HDMI circuits that are the problem.
I have seen this problem before in situations where there is something inbetween the computer/source and the TV/display.
Often amp makers will source cheap video components just so they can offer the facility without adding a huge cost to the price, but the picture often suffers.
I stopped using my amp for video switching because it wasn't worth it and the TV had more than enough inputs to just switch video sources via the TV.
Buy a proper HDMI switch if you have to use one.
Something like this
http://www.kramerelectronics.co.uk/products/model.asp?pid=1212&sf=37 2 input, 2 output
If however, you still have problems running direct with one cable from iMac to the TV, I would next try 2 things:
1) move the iMac right next to the TV and run a shorter cable direct into the TV:
- firstly this will tell you if the longer cable is the problem
- secondly if this doesn't work then it suggests your problem is either with the iMac or the TV
2) if you haven't done so already, try running direct to your projector,
- this will tell you whether its the TV at fault (if the projector is fine, but the TV isn't, then you know it's your TV)
- if both the projector and the TV show the same problem, then it's your iMac - fraid to say it, but I've seen rather a lot of Mac problems recently with using secondary displays, often it's down to either a faulty display adaptor (these break regularly) or a fault with the display chip on the motherboard and since you can't change the display card in an iMac (or Macbook) then you need to send it back to Apple.
Hope that helps.
