I own a 1987 300TDT w/120,000 miles on it. For the past few weeks I've noticed it has been idling poorly. I immediately suspected the fuel system. I replaced the fuel filter, but that has done nothing. The injectors are original. It's at a point now where it sometimes stalls when come to a stop. If it doesn't stall, it idles very roughly - worse than a few weeks ago. What's next? By the way, the air cleaner has been replaced too.