B&D GT6060 hedge trimmer?

Easy to work on removing casing one side but pain have done three times checking secondary switch. Checked brushes touching comutator, no visible lose connections.
No problem with comutator segment
Started nicely slightly slowed then speeded up to normall then stopped.
Tapped point of blade on paving it started again with occasional slight slow action stopped again tapped on floor again it started.
Stripped it down for fifth time checked all visual connections none lose checked brushes where in contact with com yes it was on both neither appeared to stick
What could be why tapping it restarted.

MANY THANKS