Sometimes can't press button on shifter

Help,
Often the button on the auto transmission shift handle is impossible to press. As a result I can't get it out of park. It seems to be better once car is warmed up. It's a '99 Cabrio. Any suggestions ?
Thanks.

As long as you can hear the shifter lock solenoid clicking I wouldn't suspect a bad brake switch. More likely the pin that releases the lever is sticking or needs adjustment.