It sounds like you have an issue with the timing and not an actual issue with burning the material (anymore). Without knowing about that machine, yes plastic needs to be heated relatively slowly. Without know exactly how fast it is going now, its hard to say if you have a real issue or not. Upper and lower heaters will always speed things up as will thinner gauge but you may still be getting the best result for your set up. Depends what you're doing and how much and the value to decide whether the upgrade makes sense.