[Anker]

Actually it is consistent. When the engine is cold the mixture is richer and the spark resistance is lower. As the engine warms up the mixture leans and the spark resistance goes up.

Under low loads, including no loads the resistance also is high.

All that you describe is consistent with the spark going around one or more plugs.

I had a Buick with exactly the same issues, took ages to convince the garage that the problem was in the spark plug wires. They kept insisting that it had to be injectors. When they finally gave up I had them replace the spark plug wires and bingo, the problem went away.