not a mechanic but...
When running your engine out of water, are you running water through the engine to cool it?
Is is running long enough that you see it spitting out of your exhaust?
Maybe someone more mechanically inclined could comment on this thought...
If you had a crack in your exhaust manifold jacket, you would not necessarily see it and it might cause problems where water could get into the engine. If you are running your engine out of water, not hooked up to water or it is not being run long enough for the water to run through the system or to build up adequate pressure to cause a problem it would always run good out of water, (if that was the problem)
I had a crack in my exhaust manifold on the exterior and it would leak water when under way, I replaced it at not much cost or trouble myself, I was told a crack on the inside of the jacket would have been worse.
Or it could be some other cooling system related issue where there is a leak, where water is going somewhere it should not.
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