fast idle when 5.3L is hot

I have a 2000 Silverado 1500 automatic with the 5.3 liter engine. The warm engine will start normally, idling about 500 rpm. Often when driving in hot weather (90F), or at altitudes > 3,000 ft., when I stop with the engine running in neutral, the rpm will stay at about 1200 rpm. This makes me use the brake at traffic signals to keep from going 5 mph.

If I turn off the ignition, remove the key, open and close the driver's door, start the engine, the idle is normal. No amount of reving the engine will case the fast idle, but driving 1/4 mile and the fast idle will often return.

This problem has been getting worse for 2+ years, but very much more frequent in the past 6-8 months. It does not happen all the time. The cold engine starts and idles normally. It usually behaves in cool and moderate Calif. weather.

Any ideas? My regular, long-time independent garage mechanic said that the plastic intake manifold was "porous" and expanded in hot weather. That allows air to be sucked in and the extra air fooled the computer. Replacement $850. He said he sprayed water (spray bottle) on the passenger side of the manifold and the RPMs dropped, and he could see steam coming out the exaust. I cannot reproduce this.

Re altitude, at 70 degrees F, altitude 7,500 ft. in Wyoming for many miles there was no fast idle. But at 85 degrees and 3,500 ft. in CA the problem occurred.

I have no reason to disbelive him, but it sounds so strange. Google web and groups searches do not locate anything like my problem. Can anyone tell me what the problem may be?

Thanks.

your mechanic is right the plastic intake is expanding when hot .

Chevy service department told me the intake manifold leaking story was BS.

I must agree because replacement of "throttle position sensor" near front of driver's side of intake manifold resolved the fast idle problem.

Is this a case of independent mechanic supporting another independent mechanic's bad diagnosis?