cannot downgrade

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http://forum.xda-developers.com – hi guys, got a u20i here, which the previous owner installed 2.3.7 cm7 onto. im trying to get it back to stock (official sony firmware, stock/rooted doesnt matter but 2.1) im having a bit of trouble though. ive tried using flashtool with a stock ftf, ive tried with an update.zip and flashing through CWM but all to no avail. im seriously at my wits end here. ive never used SE droid phones before, so this is a bit overwhelming. can anyone give me any pointers, or some sort of idea of where to go from here? thanks edit- just using pc companion to 'update' the phone right now. hopefu (HowTos)