We have a john Deere 6415 tractor. It drops the rpm's and doesn't go back up. When you crank it and take the battery cable off it will kill the tractor. We leave the battery cable off for 30 seconds or so. Then put it back on and crank the tractor back up. It will run fine for a few minutes and then it drops the rpm's again. It does this every time. Any ideas why it does this? We have changed filters and fuel lines.