I have a Nissan Frontier, 2002, 4 cyl., manual 5 speed transmission. It seem to slip out of fourth gear, and only fourth gear, especially if I try to drive aggressively, or if I try to accelerate from fourth gear. It happens most when I am driving at lower speeds in fourth gear. Also, the colder the morning, the more likely it is to slip. IS this a transmission problem or a clutch problem (or something else), and what needs to be worked on or replaced- i.e. a whole system, or only a select couple of parts?