high engine idle when in gear.

While driving at road speed then taking my foot off the gas pedal the truck idle runs at 1200 rpm and approximately 55 kph until I use the brakes to slow it down or stop. After stopping, the rpms drop slowly to 650. At the 1200 rpm speed I am constantly braking to stop catching up on other trafic. I expect the brakes will wear out prematurely. The check engine light code is 507 so I have replaced the IAC valve but it mades no difference. Any ideas.