Strange Problem

[Jim Brown]

Another item to check that I'll add to the topic, not sure where op is with all this but maybe it helps someone in future... look for a leak at the throttle body where the steel shaft (that the throttle cables connect) goes thru the aluminum throttle body. Start the engine and use a smoke machine, propane, or carb cleaner on a cool engine to see if there's a leak at this point. I once rebuilt an old Kawasaki Mule engine, put the old carb on it, and it ran like crap, drove me nuts looking for the problem. Couldn't get the idle to settle down (surging), overheating, plenty of power. Rebuilt the carb, finally traced it to the worn out hole where the throttle shaft went thru. It was so worn I shimmed it with some aluminum from a coke can and it ran like a top.

 

[studabaker]

I have had a very similar problem and have noticed it happens when it is at temp. leading to iat sensor : intake air temp. The crazy part is that I rotated the sensor to have the clip on top and it appears to have improved performance but not tested much yet. if it is doing it when hot then look at iat sensor orientation may be 90 degrees off.

 

[JVinOlathe]

Thanks for all the help. The issues have all been resolved as follows:
Upon further inspection, I found the the passenger lower trail arm had rusted away and broke and was causing the fishtailing/drive-ability issues. Further inspection showed the driver side upper trailing arm was also broken. Truck was towed to the shop and all 4 trail arms replaced.
They also found the vacuum hose under the throttle body was collapsed and creating the other issues. Truck now rides and drives like new; as a bonus, the brake light indicator that was on all the time, which at times I could get to go away buy pressing and releasing the emergency brake, is now also fixed.
Al is well that ends well. Thanks to all that contributed, your help was very helpful in diagnosing and fixing the issue. I like my Expy all over again.