1992 Maxima - low mpg, knocking during driving, idle searching

I have a 1992 Nissan Maxima SE with 140,000 miles on. Recently, engine shut itself off twice while driving but I was able to restart it. Had the engine diagnosed 4 x (twice by the dealer and twice by another mechanic) and was told that engine was running fine and that they could not duplicate fault (No fault codes either). I have had the engine cleaned as well as the Throttle Body. In addition, completed 2nd tuneup in 2 yrs. Current problem is that the car is getting about 18 mpg in the city 0 about 2.5 less than normal and that the engine seems at random times to be starving for gas. For example, will be driving down the street at 35-40 mph, 2000 rpm and the engine will drop to 800 rpm with the engine sounding as if no fuel is getting to the engine and then a second later, the car responds as if nothing has happened. It also sounds rougher than normal. Still has normal pickup, though sometimes sluggish. Additionally, getting a little knocking at various times, usually at the lower speed levels and gears. Lastly, idle at times seems to be searching for itself. Normally idles around 700-800, but is now idling anywhere from 700-1400.
Fuel used - always 91 octane or higher
Oil Change and Preventative Maintenance - always at 3000 and on time
No other major problems with engine since I owned car.

Hi,

I had a similar problem. I think after I steamed my engine that water on the distributor cap caused the problem. After a few drives it would go away. It only happens again if It rains really bad. Also, I had a plastic water sheild missing on the bottom of my car. I replaced it with one from a yard and the problem hasn't happened again. You also might wanna try Maxima.org because I saw a post relating to this a few days ago.

Good luck,

Matt