'91 500 SL, 61K miles - ACC PBC unit occasionally is unresponsive at engine startup - none of the buttons function and climate control fails to engage in any mode. Restarting the engine makes no difference. However, several hours later the unit again becomes functional. When queried the unit states no fault codes are present and will display current sensor valves. Blower motor and regulator are new and all ceramic and strip fuses have been replaced. Dealer states these early units were troublesome and have been replaced by a new style.

Is this symptomatic of a failing PBC unit?