5D long exposure noise problem(?)

I have been using my 20D for astro for a long time, and it's always performed well. my new 5D seems to no be performing nearly as well on long exposures. for example, a 20 min ISO 100 will have more noise than my 20D's hour long exposure. a 10 min dark will show almost complete noise (mostly chroma). i am considering sending it back to canon for possible repair, but i would like to hear other's experiences first. i seem to get more noise than the 20D even with the NR (c.fn. 2 ) on.

is anyone here using an unmodified 5D for astro and finding similar results?

thanks for your help

Drew, can you post a pic of a dark? I'm curious to see what it looks like.

Canon is usually not responsive to that- My previous 20D had noise issue, they said too bad, it's not meant for that kind of exposures. The dealer reluctantly took it back and exchanged a new one (it was only a week old then).

Ivan