I have a 99 cummins that lopes at idle whenever I stop and push the clutch in. It does this for 20-30 seconds then smooths out to just a rough idle. It has a lack of power and runs a little rough at highway speeds. It has 188,000 miles on stock everything except a fass fuel pump and filter system installed 400 miles ago. Any idea on the problem? Thaks for your help in advance.