my 2001 sierra 2500hd 8.1 litre has been acting up lately. it will run fine and then suddenly stall.. sometimes it will start up right away sometimes it will take 10 minutes or so. usually will do it at idle but sometimes if im going so up a hill. could this be the fuel pump? if so where is it located on th truck. also sometimes it will shut off while driving but start right away without me even turning th key.