50C on heavy load isn't too bad anymore. But there definitely was something wrong if it had idle temps of 70C. (as far as I know the conroe shouldn't go over 60C in any situation)
But I don't know if there is anything you can do, I'm not familiar with the socket 775 stuff. But you can always try everything
. My heatsink broke once so I attached it with duct tape and I got reasonable idle temps. So you can experiment with different stuff, just make sure that you don't brake anything and if the temps go over 60C then I'd just shut down the computer, you don't want to damage your cpu.
Also if you have warranty left you should find out if they will replace the heatsink for free.
Those MacGyver solutions might work, but it's definitely not recommended to keep it running like that forever, for me the duct tape was just a temporary solution (2-3 weeks or something like that) so I could use my cpu while I was looking for a new hsf.