Still it was surprising to see water literally spurting out of the condensate pipe,
By 'spurting' I asssume you mean the condensate siphon emptying and releasing a cupfull of condensate every few minutes?
My interpretation of the evidence presented so far is that as the system heats up, the pressure rises to the point (P_hot) that something acts as a pressure relief valve. (This could be the 'official' PRV or if could be a cracked joint somewhere, with the HEX as a likely suspect.) The system will then operate happily like this (at P_hot) until it is allowed to cool at which point it will be apparent that water has been lost (pressure drops to P_low).
This would be confirmed if the next time the system heats up the pressure starts at P_low, rises to P_hot again and then drops back down to P_low when cold.
As others have already suggested, an empty/faulty expansion vessel and the PRV are the most common cause of such symptoms.