Sounds like your battery is stuffed. If the best you get fully charged rested voltage is 12.3v then it's only holding a 30-40% charge. Lead acid batteries will not operate well at that level. There may well be other issues that have resulted in the shortened life of your battery.
Download
this PDF from Yuasa about testing your OEM sealed VLRA battery. Note the charge percentages x voltage table. Lead acid batteries don't like going this low - don't ask me how I know.
