I have a 84 300D that has been having trouble accelerating over 60 mph on the highway. Now it is having problems at lower speeds as well. The clincher is that this only happens on my normal tank, but when I switch to my second tank (for vegi oil) the problem disappears. I realize the problem must be downline of the transmission, engine , etc. but I cant figure it out. After replacing the diesel tank feul filter it still exists. Any ideas? Also, maybe unrelated, when I accelerate at times there is a gun type sound that fires at regular intervals out of the muffler. The intervals increase when acceleration is increased.

Thanks for your help.
Sam