A couple of things spring to mind,
1) one the motor, knock off the power at the plug and with the blade raised off the table and forward so it’s not touching anything, spin the blade, does it spin freely with no binding ie are the bearings free spinning, I’m thinking of a seized bearing.
2) The motors normally are very long lasting, but I guess everything is prone to failure
a) Check the motor reset button. Is it tripping ie before it should? They are bimetallic strips? And trip off like an electric kettle does when hot.
b) or the motor windings have a fault
3) For me the more likely and I hope, as it is a simple but pricey fix, is the on off relay switch in the handle. They did have a tendency to fail, I bought a nearly new DW 1201 with a no start problem, my switch cost £75.
For the checking of the items, i’d refer you to the many helpful experts on this site who would advise on the more likely of my suggestions
re the setup of the radial saw, DeWalt used to have manuals readily available from their website, I haven’t looked recently but the pdfs were readily available at different sites like this one on the web, a Google search should prove successful.
I am comfortable using the radial saws and have had them for the last 30+ years. Setup for smooth safe cuts is key, as is a sharp blade. sorry chaps but I don’t buy the cutting out due to poor adjustment unless the blade it is stuck within a joint. And according to the OP that is not happening.
I don’t think the DW 1201 has an electric break, it is too old for that requirement.
if it was the windings I’d of thought it would have tripped the consumer units earth leakage trip. Is there one fitted?
I'm convincing myself, the more I think of it that it is the on off switch. They are complicated devices, if the relay inside is letting go under load then it would switch off and give the described symptoms. sadly they were pricey, again maybe someone here knows how to check them? I just bit the bullet and bought one.
if the OP can’t find a manual and wants one please message me else I’ll forget
and I’ll upload one if allowed. Cheers Graham