It sounds like an electrical problem to me. A wire maybe be frayed or partially disconnected, and therefore, is cutting out the power when it's no longer making contact. Unfortunately, it sounds like you may need to bring it in for repair or call an electrician friend who may find the problem.
Have you tried pushing the peg inwards?
This locking peg should be spring-loaded, and tapered at the top to retract automatically when the handle is pushed down, like a folding umbrella.
However, it could be jammed, so try manual force.
The lock-peg may have trapped a piece of debris around its body.
The on/off power switch faulty. The switch gets used every time you use it.So It might be that.Unplug it and open the switch.You can use a meter to check it.If it's in the off mode and still sends a signal...it's the switch
Suction stops when there is either something blocking inside the hose or you vacuumed a sharp object that created a hole or tears in the hose or bottom transfer line.
You will need to look very carefully at the hose to see is something is jammed up also turn the unit over and look at the looping hoselike section to see if there is a tear or jam. if its a jam carefully remove the object. if its a tear you will need to get the part from Dyson to replace it. since the unit is new you can also send it in for repair. depending on whether the unit is new or refurbished you should have from 1-5 years.
This is common on the 25's. Take it to your local vac shop for repair. The pivot screw vibrates loose and binds the pivot point. This can be fixed. You probably need it serviced anyway as bagless machines need annual cleaning to perform properly.