96 850 turbo RPM on idle

Im asking about a friends car. Its an automatic 96 850 turbo. I'm not sure of the miles. Over 100,000. When the car is idle it will be at the normal rpm's then all of a sudden it sounds like someone put there foot on the gas and the rpm's go up a quite a bit. Not to sure how many. My friends also says it happens when she is driving. But if she does have her foot on the gas when it happens it will actually make her car go faster or something. I thought that kinda sounded weird. It happens to my car if its idle but it only goes for about 1000 to 1300 rpms and doesnt get all loud like I have my foot slammed down on the gas. Just wondering if anyone knew what this could be.