Do you think it may be overheating?
There is a "bypass tube" (#8 in service manual cooling system diagram). If t is leaking, it WILL cause overheating.
If overheat is occurring and the engine is shutting itself down as a result, you could be damaging your engine by continuing to run it.
Repair that leak before running it anymore.
Give us some background on how this came about. Did it run great for a long time and just started doing this?
Or, is this a recent purchase? Those sorts of things.
If you get the water leak repaired and it still wants to quit running, try it with the engine cover off and see if that makes a difference.
But, whatever you do, keep a close eye on the temperature. Buy a cheap, infrared, non contact, thermo gun from Harbor Freight Tools . Well worth the money to protect your motor.
This engine should run at no more than 165 to 175 degrees Fahrenheit.
Let's see if we can get her straightened out.