keeps stalling
 

it will start up fine and idle fine. but when i start to drive it sounds like it wants to stall if i dont give it some gas. when im driving down the road and keep it at a steady speed, it acts as if im giving it gas and letting off and repeating it. to me it sounds like the timing is wrong. i had to fix the timing before and it was diving fine. could it have changed the timing some how?