I usually go 1) click bootstrap Recovery 2) after the success! message click Reboot Recovery 3) Wipe data , Wipe Cache ( +Dalvik cache in the advanced options) 4) install zip from sdcard ( and then select the file(s) to install 5) Reboot
If you don't wipe data/cache you can go into bootloop easily.
P.S. I have never used rom manager to install roms so i can't help troubleshoot that part
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