Our car started stalling at lights, stop signs. To keep it running I had to keep my foot on the gas and brake until the light changed. Once it was going it was fine until the next stop and I'd have to do it all over again. At highway speeds the engine lopes. I had it in the driveway and revved it until it started loping. It would go to 4K then drop to 3K, go right back up to 4 drop to 3 This would continue until I let off the gas. The check engine light is not on and there are no codes read on the code reader. Any ideas?