then it acts as if its running out of fuel and stalls. in town driving will consist of stalling at every light i have to stop for, truck will start up again with no hesitation what so ever. when its under a load ( pulling a trailer) it happens faster . when the outside temperature is under about 65 it will not react the same way even while driving in town. seems it has something to do with the ambient temp/engine temp. Have had complete recent tune up, replaced radiator, thermastat, cat converter,map sensor , idle control valve but to no avail. trk has 138000, this began about 100,000. does not throw any codes at all. just stalls and restarts every time until it has to sit at the next light , stop sign or any slow moving situation. when i get to my destination and it sits for a bit its ok until it warms up again. can always hear fuel pump running.