V-10 engine in a Ford E-Super Duty Class C motorhome. After sitting, the fuel pump relay "chatters" when the ignition is turned on. While this continues, the engine will not start. After repeated cycling of the key from on to off, the "chattering" may stop, at which time the engine will start and run fine. Immediate restarting is not a problem. The "chattering" is rapid cycling of the fuel pump relay. I have swapped the relay with another and the "chattering" continues. If I let it, the "chattering" would just keep going. Any suggestions on the problem?