1999 Chevrolet Silverado • 188,000 miles

I have a '99 Silverado 1500 with a 4.3L V-6. It picked up a problem recently that's been stumping me. Occassionally, and for no reason I can sort out, it starts to shimmy badly. This behavior has come about at several speeds. Sometimes just cruising down the road at 35-45 it starts to shimmy and wobble and the steering wheel shakes as if a wheel is horribly out of balance. Other times this occurs at highway speeds - it's noticeable and alarming. The problem can last a long while; othertimes the symptoms disappear in a few seconds. The first time it happened I thought I had a flat or was about to lose a wheel.

I've checked several things and came up empty. No delamination in the tires and nothing out of round, but just to be sure I had the wheels balanced and rotated - the balance was spot on when checked. Alignment is good. Did those things, but the symptoms continued. The vehicle has 188,000 so I put in on the lift and checked the front-end. The steering rack has a very slight amount of play and the hub bearings have a very small amount of play too - I'll get after those, but they're not causing this intermittent problem. Everything else is tight. Shocks are fine, brakes are fine, I checked the rear-end as well - everything is straight and true. Air pressure is set correctly. I checked the driveshaft for runout and play - that's not the problem. The truck has never been in an accident and nothing seems misaligned.

The truck drives like a new truck most of the time, then suddenly, on a smooth road, it starts wobbling around. I'm starting to believe the problem might lie under the hood. I had a problem some time back with misfiring and the truck bucking - this turned out to be a worn distributor. A new distributor, rotor and wiring solved that problem. But these symptoms are not similar - this is not a bucking problem; the entire vehicle starts to shimmy and wobble with the steering wheel vibrating back and forth. It's hard to tell, but I cannot detect any misfiring or rough running in the engine when these symptoms appear and I have no 'check engine' warnings.

Are there any engine/transmission/drivetrain related issues that could cause intermittent symptoms that feel like an out of balance condition?

A few other facts:
- I have noticed the truck is a little down on power lately
- The fuel economy has dropped off - I used to do around 20, now I'm down to 16mph highway
- (Very) slightly rough idle

Thanks for any advice you can give!
January 22, 2013.

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