I have a 97' ford f-150 with a v6 4.2liter engine in it. its got like 166k on it but we went through and redid the motor at 163k. The problem is that every now and then it starts to run lean and causes a lean miss fire. I have checked the vacuum hoses and I did find one lil rubber boot that had a hole in it but I replaced that and it still runs lean and miss fires. And all the miss firing is on one cylinder. Any Suggestion?