I just moved into my first house. 1950 Colonial with an old Stewart Warner boiler which uses oil, with hot water radiators of course. I first had a problem with the safety relief valve but resolved that by draining my water logged expansion tank.
Now I've notice a much bigger problem. My boiler will go on for about 4-5 mins, then turn off for 3-4 mins. The thermostat is set to 68 and its reading a current temp of 55 so its shutting off way before reaching desired temps.
On the boiler it reads around 12-15 psi when it turns off, which I understand is good, but the temp is 190 and it seems to go off when it starts going over 190. What am I missing?? Is there a valve not opened.

What am I missing, and how do I fix it?
Also, what should be my low/hi aquastat setting for this brand boiler?
Any help is appreciated. Please keep in mind this is my first home and first boiler I've ever had to try and fix so please excuse ignorance.
TonyV53