Question Asked: what will make my 99 e350/v8 run poorly after it warms up? Marc

sounds like you may have a intake problem, i have seen this happen several times, when the engine gets hot or warm the intake maybe warping and sucking too much air,try spraying carb cleaner,break cleaner, or some sort of flammable spay and spray around the intake area when vehicle is acting up and if the rpm's increase then you can locate the problem.Hope this helps if you have anymore questions please email me, Rick at watsonautoservice@gmail.com