One minute truck is running fine, next the service engine light starts flashing, no exceleration and popping noises and can't get above 1200 rpms. Pull over let sit for a bit then only light is on and no symptoms for awhile then it starts all over. Weather is damp.

 

Flashing MIL usually indicates a misfire code. Pull the codes and see which is the offending cylinder(s)

 

Thanks, will check codes. The SEL has been on for years, the flashing just started today. It is a wet snow outside. Last week it seemed to run rough and again it was wet out.

 

dampness can effect the spark plugs / wires.. but normally after it warms up a few minutes that will go away in humid/ wet weather.

 

and can't get above 1200 rpms. Pull over let sit for a bit then only light is on and no symptoms for awhile then it starts all over.

The engine is probably is in fail safe mode at 1200 rpm's. the engine will limit its rpm to protect it self if the computer recognizes a potential engine malfunction. Yellow light is caution, red light is severe damage. All modern vehicle have a rev limiter when its an a critical stage. by your description the motor is going rev limitations to protect itself. lack of oil? lack of coolant?

 

Nah, just a severe misfire.

 

My 03 did the same thing, was cylinder 7 misfire, replaced plug and coil pack and now runs ok, I do need to change the rest of those plugs soon.