1998 Ford Escort Idle and power loss

Runs great until it gets warm. It used to just idle high after it got warm Usually sitting in traffic makes it slightly warm. Then, after driving for 15 minutes, when you slow down and press clutch, the rpm's stay very high. I shut the car off and the rpms go back to normal until I get up to a decent speed again, then the rpm's stay high again. Now in the last 4 weeks, when the temperature gauge goes up a little bit, the car acts like it is going to die. When stepping on the gas it feels like it will die, so I have to get rpm's up. So, to summarize, idle doesn't return after car gets warm, unless you turn it off, and the car begins to die when it gets hot from sitting in traffic.