[Laird Scooby]

What sort of specialist did you take it to? They should have done a cooling system pressure test.

Is it automatic? Also have you checked the engine oil level and have you seen evidence of any water/coolant leaking out anywhere? How much coolant did you put in after fitting the new rad?

Loads more questions, these should hopefully give me a few clues which direction to investigate first.

As for K-Seal - it's a last resort and it may have clogged something that needs to be open.