Engine stumble/hesitation issue

New development with my poor 2600--I now have a stumble or hesitation from 900-1300 rpms if the truck is under load (starting in 1st, accelerating after braking in that rmp range etc). Acts like it's going to cut out a couple times, stumbles, and then as soon as the rpms are over 1500 runs smooth. Didn't do this before, any ideas?
WOT has no issues, nor does constant speed at 1500rpms plus.

sorry its been a few months man hopefully u still check this once in a while. try cleanin ur egr valve right under the carb on the intake on the passenger side. it gets real gunked up and check your plugs too. im on my second mazda and the dirty up um plugs if they got some miles on them. ive had to clean both egr valves on both trucks, did it the other day and cleand plugs and it idles sooo much smoother.