From an earlier post you indicated that sludge in the system had been dislodged by Sentinel X400. Do you have a magnetic filter on the system return? If so is it removing magnetite?
Before you replace the boiler, run it independently by opening the heating valve, bridging the thermostat input and running the system on the boiler stat. If ok, then address the control system issues.
The best approach is a simple logical process to find the gremlin. Initially, eliminate the boiler as the culprit. From your postings, I doubt it is the boiler, but sludge in the system does not help with the diagnosis. Eliminate this as an issue first
Before you replace the boiler, run it independently by opening the heating valve, bridging the thermostat input and running the system on the boiler stat. If ok, then address the control system issues.
The best approach is a simple logical process to find the gremlin. Initially, eliminate the boiler as the culprit. From your postings, I doubt it is the boiler, but sludge in the system does not help with the diagnosis. Eliminate this as an issue first