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I'd suggest the next step is to do a compression test across all cylinders as they should all be within 10-15 psi of each other
as this will show if the gasket is still sealing correctly
if leaking then this can give issues with starting
The other thing to try is a head gasket / block test kit as these take a coolant sample and analyse it for any signs of combustion gas - if present this would indicate the head gasket is on its way out