1, stop unrooting. What recovery are you using? If you're still using CWM, get Amon Ra form the Development section and use that instead. If you have ROM Manager, don't use that to flash ROMs. It's really a pretty useless app.
With Amon Ra, go to Wipe and wipe everything except SDCard: and Battery Stats, if you're not fully charged.
New ROMs need a lot of space, so look up the guide for partitioning, also in the Dev section and partition your card before you flash the ROM. Make sure the ROM you flash has DT A2SD baked in, or else, get that file and flash it in the same session you flash your ROM in.
And remember, it takes upwards of 10 minutes for some ROMs to boot up the first time.
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