Ebike repair: motor stutters on start and thumbthrottle only works now and then 

In 2015 we build our first ebike. It was a Sparta Maxx 70. We placed a 48V frontwheel hub motor. The Silver fish battery was replaced recently.
Now we have a problem where the motor intermittently reacts to the thumbthrottle. Also the motor has problems spinning up to spee. Is it the controller or might there be another cause like the wiring?
Watch the video and see how we analyse the problem and discover the real cause.
music by Paul Stamp
tracks: new world 97&nutshell
/ @paulstamp9025