Truck Repair: m-11, head gasket, alarge

Question
hi there.Occasionally i get a alarge shaking in the tractor with a loss of power to the extent that it will just maintain headway on level ground This apeers to happen in the top three gears.most time it goes away within 2 minutes.some times go dayswithout aproblem .sometimes aproblem all day.motor is m-11,made in 96[370e].under 500k miles .have replaced 1 injector and head gasket in last 6 months.not uncommon to drop a gear which forces up rpms problem goes away with foot on the floor.any   ideas what the problem is THANKS knud

Answer
Any "Check Engine" light coming on?Might possibly be an injector solenoid sticking under a load.takes a little time to find which one it is is the only problem.Have to hook a laptop or hand held diagnostic tool to it & drive it while it is doing it.Then cut out the injectors to see which one it is.