What is wrong with my car?

I have a 2003 Nissan Altima 2.5 S with approx. 78,000 miles on it.  Periodically, while driving on the freeway or on city streets, it begins to jerk and the RPM gauge goes crazy for a second.  Any idea what this problem could be?  When the mechanic drove it, it didn't do it.  The check engine light is not on.  I thought it might be bad gas but I put in Chevron gas and it still did it.  Any ideas on what it could be?  I contacted Nissan and they said that a recall had not been performed on my car for the ECM.  This will be performed in the morning.  When I spoke to the mechanic, he said that wouldn't be the cause of what I'm describing.