Doug -

You can check the intake manifold by disconnecting the air hose from the throttle body and having a look inside. If it's pretty clean, the manifold is likely OK.

The IAC can be intermittent if it's glopped up. There's a way to clean it with carb cleaner, look in the 700/900 FAQ's. Just don't get the cleaner up in the electrical part.

>>A shop said that the problem was a dirty intake manifold. But--how come it is only dirty sometimes? <<

Another question could be "Why does a dirty manifold create a problem sometimes and not others?"

Check other things, too. Is the problem more likely when the tank is less than half full? (Remember that the needle at 3:00 position means nine gallons +/- used, not half full.)

Good Luck,

Bob

:>)