Sudden RPM Spike

[rwp]

Greetings, I am having a problem with my 300SDL, occasionally and becoming more frequent, while at idle the rpm jumps by 500 rpm, it will then go back to normal. its like a sudden surge in power. The whole car kind of jumps up and it feels like the transmission has a surge as well. I have only experienced this surging below 1000 rpm. As of late the problem is increasing. Yesterday it happened three times while waiting at the bank. When stopped I am putting the car in neutral as a way to monitor the surge. Any input would be appreciated. My timing belt is in good condition.

Thanks