2000 Ford Ranger

2000 Ford Ranger w/ a 3.0l Flex Fuel engine. Check engine light came on one day. Checked codes. Said #6 cylinder not firing. Took compression check of 2, 4, 6. 120, 125, 85 respectively. Noticeable skip. Fuel mileage very poor around 13 or 14 mpg. One day we unhooked negative cable from battery and left it for a few hours. Hooked it back up and after a few minutes engine ran great with no skip and did so for about 2 days with great fuel mileage, around 21 mpg. We have done this procedure unhooking the negative cable, leaving it for a period of time and rehooking it up, the truck runs great for 2-3 days with great mileage and then reverts back to one cylinder skip which is #6 as well as bad mileage. The truck is in good shape and is worth fixing, but if we fix the mechanical problem it seems there is something electrical or computer oriented which we should fix first but we don't know where to start. Any help would be appreciated.