Renault Laguna 1.9 Daimonique 2005 Paking brake intermittent fault?

The warning light comes on in the dash board and when you stop the parking brake comes on and won't release, so you can't dive away. If you turn thr engine off, it is OK gain for a while. This is a very intermittent fault. Has been in Renault garage and they can't sort it. They have reset the parking brake computer - thought it was fixed, but happened again. Had new computer fitted, but it still happens. Garage are stumped. Any ideas on what could be wrong and how to fix it?