2002 Toyota MR2 Low idle & shaking once warm

My cars starts just fine, but as soon as it reaches normal operating temperature, the idle drops to about 500rpm (from about 1100rpm), and the car starts shaking. This happens only at a stop, but it happens as soon as I stop. This is a new problem that started suddenly. There are no codes in the computer, but the oxygen sensors were sending different voltages, so I replaced the two oxygen sensors in the header (did not change post-cat sensor). Now they read nearly identical, but I still have the idle problem. Toyota claims it is the computer mapping, and that I have always had this problem (and that it is not a problem), but I would have noticed this in the first 100,000MI if it was there before. I tightened the throttle cable so that it cannot go to idle, but I would still like to find the cause of the problem.