I've been having some trouble with the master keys for my '98 GS400..

When I press any of the buttons on the master keys it only works like 20% of the time. The key batteries have been changed and the red light on the key comes on every time. Also every time I go to the mechanic the key starts working every time.

When I put the key in the ignition the engine starts every time.

When I use the powerlocks on the driver door or by putting the key in the door and turning it, it works every time. I think that means the actuators are ok.

How can I find out which component is bad?

Update: I think there might be some radio interference where I live. The remote key seems to work normally when I'm away from home.