Occasional rough start up/idle

1984 SR5, 4 x 4 with 22R engine. Sometimes when I start it the engine, for about 3-4 seconds runs like it's running on 3 cylinders and if I rev it up it then smooths out and runs fine. The temperature outside is about 72 degrees because it mostly does it when I'm leaving work.
There was 1 day when the idle was rough all day, the truck was shaking when I was at the stop light. The next day no problem at all. I took it in to the mechanic and they checked it out throughly on the diagnostic and could find nothing wrong.
This morning it started rough again for 3 seconds but then smoothed out fine.