Ok, a few of us here in Texas have ZC motors in our Rex's we have had that wierd up and down idle problem for a little while. Figured I would share the results.
On the throttle body there is a vacume line. Its right next to the throttle linkage. Pull the vacume line off you will hear the idle go up and let it run for a second and then the pitch will come down as soon as it comes down slide it back on slowly. Your idle will be solved. This has fied three different ECU's that were doing it. The idle was only jumping like 2 to 3 hundred RPM doesnt hurt anything just annoying as hell.
So this is the fix have fun