[thstone]

Great looking car - that paint is beautiful for a 15 year old car!

What did the mechanic who did the PPI say about this problem? That would be the most trusted information.

I hate to say this, but relying on what the seller told you is .... well, let's just say that the information he provided might be questionable. I say this because if the problem had actually been diagnosed down to one particular "sensor", then he would have told you the exact part number that was needed to remedy the problem. Even more likely is that he would have fixed the problem before selling. Since he didn't provide this info, then he probably had someone look at the car for 2 mins and they said, "Its probably a sensor" and this is what he passed on to you.

For reference, 99 Boxster's have a throttle cable, not E-gas, so there is no TPS (throttle position sensor).

The idle control valve mounts to the side of the throttle body and maintains the idle RPM's. The ECU will adjust the idle RPM to account for the added load on the engine when the a/c is on.

In my experience, there are a number of components that influence idle RPM's and might be the source of the symptoms that you are experiencing (MAF, ICV, TB, air leak, ECU, temp sensor, etc).

My suggestion would be to have a competent mechanic look at the car to find the source of the issue.

Maybe someone else has had this exact problem and can help with a more specific fix based on their experience.

And if you were/are a US Navy Seal, thank you for your service!