P.S. I too have had a lot of problems with using RM to install ROMs. I only use it to update CW and re-flash CW when it breaks.
ROM Manager is very fickle. There are a couple of steps you can do that will make RM work better for you.
First and foremost....... always clear the download cache in RM. To do this go into RM > menu > clear download cache. I do this everytime I want to do anything in RM.
Second, whenever possible, I run WiFi when downloading trough RM. Using WiFi instead of your 3G makes RM download faster and more complete. Less changes of a corrupted download.
I have been using RM since I rooted and have had only a few minor problems with it. Once I started do the above 2 steps, no more issues with RM.
i'm having another problem and i was wondering if anyone could help me...backing up using sprecovery is working fine but when i try to backup using rom manager, after i click backup my phone reboots and then when i open rom manager it says recovery failure. then it gives me three options; recovery mode is broken, rom manager doesn't reboot, and other. does anyone know why this may be occurring?