k800i and w580i problems help please

Hi new here, I have a k800i works fine when charged, then after a day it switches off ?. when swithed back on it only stays on a couple of mins then switches off again.. tried a new battery but still the same ?.. the 580i when i close it it comes up saying battery low when i open the phone the battery meter fills up again, battery is fully charged ?. any one any ideas.. many thanks..

fully reflash both then see if problem still there