I recently did an oil to gas conversion. Burnham V74 to a TT-110. Use the same 17 year old 40 gal Amtrol boiler mate. Piped it like it was ( w 007 on return) and got a call from customer saying "used to be able to get 4 showers now only 2 what gives? Tried a new OO-R w ifc and installed it on supply side of dhw coil and removed leaky swetcheck from return pipe. seems to be ok.... circ is on medium speed and dhw priority is on and working. if I get a call where would you go , other than replacing indirect?
Are you piped primary/secondary like the manual. What temp it the boiler producing on dhw. Is the boiler going to high fire?
Those amtrols have 2 issues.
The coil has very high head loss. This could be causing low flow and preventing the tt to reach high fire.
They also have an undersized coil that is prone to calcifying on the dhw side.
If you verify that the boiler is piped correctly and is reaching high fire, I think it is time for a new tank.