It sounds like you have a HW coil in the boiler. What you really need is a tempering valve on the outlet of the boiler for the potable hot. Because water in the coil will get to the boiler temp when it sits there for awhile, the initial shot will be that temp. You need the tank to be hot so that water passing through it can warm up if you run more water through than is already hot. Lowering the boiler static temp decreases how much heat can be released to the water passing through it. You could try lowering it, but may find when using more than a few gallons, the temp changes and cools off too much.
This type of setup should always be installed with a tempering valve to moderate or temper the output to a safe level - it basically mixes cold in when required to keep the max temp within safe bounds.
If you have one, it could either be set too high, or it has failed and needs to be replaced. If you don't have one, install one for safety reasons.
Where I live, these are required even for tank type WH. They normally come preset to 119-degrees, but are adjustable higher or lower.