Jerking or bucking while driving

I own a 99 F-150 with 114,000 miles. Like some other questions I was scanning through, this just may be repetitive. I acquired my truck in the summer of 2004. Around November of 2005--may not have noticed the November of 2004--it began jerking during normal driving. Like others, a lot of money was spent to diagnose and correct the problem. The jerking stopped. Jumping forward to November 2006. It is happening again. Putting two and two together, it only happens during the cold weather months and when going up hill or during a strain. I live in Alabama, so only a couple of months go by with cold weather. Could this be something to do with the vacuum system or master cylinder? On that, when should I feel resistance when I apply my break pedal? Right now the pedal moves around 6 to 7 inches before I feel pressure. I can pump the pedal and it gets tighter. Thanks in advance for the help.