Best thing to do is connect a multimeter across the lambda and see if the voltage swings - should go from about 0.1V min to 0.9V max - if this is happening, then I would suspect that the lambda is OK. Lambda will only start feeding the ECU with data when the engine has warmed up a bit, but if you connect the meter with the engine cold and then watch the meter reading as it warms up, the reading should begin to drop, and then the swinging of the voltage should occur as the ECU richens and leans the mixture in turn.
Next port of call would be the engine temperature sensor, as you say the idle speed sticks high after hard driving - this would say to me that the engine is not getting the correct temperature reading and is thus overfueling (that said Lambda should sort this to some extent).
You really need to get this onto a proper diagnostic computer and get all the readings checked. If there is anything wrong, the computer will show these in the log, and if the temp sensor is knackered you'll see it fairly easily (note, the temp display on the instruments is driven from a different sensor).
One final idea would be to disconnect the battery for five minutes, this has the effect of clearing the memory on the ECU, and makes the system relearn some of it's settings - It also clears any logged events so you really should have the car on a diagnositc before you do this.
Hope this helps