My '85 240 DL (102K mi) has rather abruptly started running rough, especially when cold. It accelerates unevenly, especially in hard acceleration after a stop, or going uphill. It loses power momentarily, then kicks in again. This was getting steadily worse for 3 weeks or so, and now is especially bad after sitting idle 10 days while I was out of town. There are some "burps" or pops, but no major backfiring. It also idles rough after a cold start, and threatens to die before I get rolling.

It seems like it is either in the fuel or the ignition system. What are the most likely factors, and how should I troubleshoot?