Cadillac Repair: 96 sts cad runs on its own, cad, check engine light

Question
The engine runs high that i sometimes have to drive with the brake on. It will go about 40 miles an hour on its own. I have had it to the shop 3 times, and no one can ever fix it. it will do this about every other day. the last person changed the idle valve, but after we had it home 3 days it started again help

Answer
IS or HAS the CHECK ENGINE light ever illuminated while driving the vehicle and or has any other lights illuminated or any messages from the message display center ?.

What work or diagnostics of ANYKIND WHAT-SO-EVER have been performed on the vehicle trying to fix this problem ?.