3gs battery issues

[white573]

i have a 3gs 32g it is the first gen 3gs w 4.26 bb and i am running on 3.0 fw i have had no issues w my phone untill recently my phone will say a good percent of batt life then if i go awlie w/o using it it will show low batt life or turn off then i will have to plug it in then when it turns on it will say a high percent again i have heard of batt issues w jb but never experienced it untill now should i upgrade to 3.1.2 or look into a new batt from apple im not sure if that is even possible now or not any ideas or answers? thanks whiteI32