I have a 1999 ram 1500. For the past few days I have been driving, the truck feels as though it is stalling while driving. When I keep my foot on the accelerator just enough to maintain speed, typically at 1500rpm, it will drop to 1100rmp to 1300rpm. This is typically between 35mph and 50mph. It will violently shake and have almost a bucking movement. If I let off the accelerator or press harder on the accelerator and bring it above 1500rpms it will stop. It doesn't happen every time I cruise between 35 and 50, but it happens enough to be a concerned about it. Please let me know what you think could cause this. Thanks!

 

Are you currently showing any codes? Particularly a misfire? Sometimes these can be more apparent while in overdrive. How many miles are on the truck? When was the last tuneup?

 

A failed oxygen sensor will do that. If it's the O2 sensor, you'll note that it happens only when cruising lightly loaded and will disappear when the engine has to work harder (thus dumping more heat into the exhaust stream to warm the sensor). If you let it go like that long enough you'll eventually get a CEL and a code about it, but almost no one has patience enough to wait that long. If you've noticed that the engine runs with a stumble for a time after a cold start, usually starting in about a minute and lasting for at least several seconds before clearing up, that's enough to call it all but perfectly certain that the upstream sensor is on its way out.

If it's been more than 80,000 miles since the oxygen sensors were changed, you might just as well change them out now anyway. Use only dealer parts, or Denso or NTK brands (in that order of preference). Others, including Bosch, are as likely as not to give you grief either straight away or within a relatively short time.