I have the same problem, have had the same "solution" from a renault dealer. Basically I think the transmission is stuffed. It only happens when the car is warm from driving in city traffic. It has become dangerous in that its just a matter of time before I get rear-ended due to the car's inability to change gears.
My (elderly) dad suggested changing the transmission oil to a thicker grade, but my mechanic said that's not possible. He suggested I sell it or spend the $3-4k needed to get new transmission fitted. The car is 8 years old and has almost 100,000kms on the clock. These are my choices!
Chris, Sydney, February 2012