raypak hydronic malfunctioning controls

I have a Raypak copper-fined tube boiler used in a circulated hot water heating system (baseboard hydronic).  This fall it began to make a noise, a descending in pitch, increasing in volume "shriek" that was finally observed to coincide with the "high fire" stage of the two stage gas valve.  The shriek was significantly reduced by adjusting the two manual controls (stage to 160 and high limit to 180).  However, coincidental to the shriek was quite a bit of water hammer.  After paying several companies to take a look at it, this still remains unresolved.  I have observed that when the boiler receives a call to heat the water in the boiler (an internal call, as none of the three zone thermostats are calling at that particular time) the water quickly exceeds the high limit setting; in fact, the boiler kicks on when the boiler temperature gauges reads 185 or 190 (which should be the cut off) and continues to burn until the fast response temperature sensor control board registers excessive heat (yellow bulb) which indicates water temperature exceeding 240 degrees (thus boiling the water, making all kinds of drumming noise, and causing air that needs to be released and new water drawn in).

Question: does anyone know what is calling for the heat (at such an inappropriately high temperature) and what component to check for failure?  I have a small meter, and checked the fast response temperature leads, and got a "2" on my scale, which from reading someone else's questions I assume means the probe is functioning).  Thanks in advance for any advice!