1994 Toyota Pickup whats the deal?

My 94 toyota pickup has been acting up since this summer. When I would be stopped at a stoplight, it would start to shake a bit, and the engine seemed to be pulsating. At first, the issue seemed to be a lack of coolant-but about a month after that issue was solved, the pulsating returned. My second visit to the shop involved the replacement of my radiator. Again, my truck was good for a little over a month-and the issue returned AGAIN. Brought it back to the shop, they said a "tube" was loose-fixed that, and once again, I am faced with the same issue about a month later-only this time, the engine seems to pulsate as I am driving and the truck shakes a bit when I am stopped-and my heater isnt working. Aniy thoughts? I will probably take it to a Toyota Dealership this weekend to get their thoughts, but thought I would try you first?