The problem turn out to be the thermostat ground. It wasn't torqued down tight enough.
I however have a new problem. I only have spark when turning the ignition on and off at the plug that the distributor rotor has stopped on.
I have no spark when turning the engine over.
I intend to do a full check on the coil and ICM but it's very difficult to remove the bolt that holds the rotor due to the splash shield.
Is there anything other than the distributor, coil and ICM that might be causing this problem?