My truck ('79 K20) has started stumbling at highway speeds (55-60 mph). It drives fine up until that point, but at that speed, it starts acting like its running out of gas (stumbles to the point of almost dieing, and the more I push on the accelerator the more it does it). It just started doing this in the last month or so. If I lay off and let it slow down, its fine. Also, if I really get on the gas from a dead stop, it runs just fine. Could the fuel pump be going out and causing it to starve only at the high end? Or could it be something else? I'm getting ready to sell this truck, and want to get all the little bugs worked out. Thanks for any help!