The cable is prob starting to fray. There will be a stiff outter caseing on it that is mounted to the engine. I should be on top of the motor somewhere, alot of times it will be red or someother bright color. Take a look at the exposed cable and see what it looks like...
actually, after thinking on it for a sec. If your throttle cable was going out, it would do it reguardless of whether you were in gear or not. So, I would say it is in your throttle/shifter...