what cause my jeep to miss stutter or start shaking?

When you are driving at any speed in between 40 and 55 try to keep it at any steady
speed in between those two speeds. it wants to start either missing,boughing out,or
flat out shaking. I mean lets say your going accelerating up the speed f 50. When you
get to 50 and hold the accelerator to keep at that speed. As soon as you push the
accelerator the truck start to like wobble and shake. Like the wheels are going to or a
death shake. But if you hit the gas like you would when passoing someone and get the
rpm's to shoot up it stops shaking. An i am also having trouble with the jeep's air filter.
For some reason i am on my second air filter for this truck in less then two weeks.
When i got the motor put in it. I had a new air filter installed. Three days ago i had to
change that one cause it was so dirty and black. Well i checked it again today just to
see and it is already dirty and starting to build up black again. What could be causing
my jeep to do these things?