Why does my car speed itself up while I am driving ?

I can be driving at a normal speed and all of a sudden i can feel it speed up, its like I put on cruise control and it will not even be on..and I can then take my foot off gas pedel and still be going 40 mph and not even slow up and if I stop and put it in park it will be like 20 rpms and you can hear the motor speeding away..and the gas milage is terrible now...can you please tell me what is wrong ...and NO the check engine lite is not on ..thanks for your help.. oh i forgot to mention it does not do it all the time just once in while but its seems to be getting more frequient.