First up well done for holding your cool. Too many posts with fiery red faces as if it is all someone else's fault!
Couple of thigns I would check...
If you used lead-free solder, re-do all the connections on the speedo and the motor. I've had several crack and it's a known problem with lead-free in other applications. You could try giving each wire a sharp tug at each end, but I had one that survived that test but was still able to get a dodgy connection - after a re-solder the problem went away. Use some flux paste if you can, it helps a lot to get a good joint.
Have you checked the sensor board on the motor? You mention the sensor lead, so another place to check is the board. Try another motor if you have one rather than fiddling with the board on the one you've got. If it fixes the problem...
I am not sure this is what you want to hear this, but I had this problem once with an HW, and after that it refused to behave properly at all. Amongst the symptoms was the one you have - it just stops working intermittently. Mine went on a journey back to China and has returned in fine fettle! Contact CPD Racing if you bought it in the UK and ask if they can help. If you didn't buy it in the UK the returns procedure is on the HW website. HTH
