I have a 21 jewel waltham crescent st with an up/down indicator.And recently it will sometimes stop when laid on it's back I know there are several possible reasons for this,but I would like to know the best way to start troubleshooting the problem. Watch is running fine in all other positions. Any ideas?. Thanks.
Karl gave you the outline for the procedure. What I do is:
Inspect all escapement jewels and the staff especially; under a microscope.
Ensure escapement functions (lock and slide)
Remove EW and pallet. Install balance complete. Does it maintain its swing in all positions
Remove Balance Spring and repeat
Install pallet bridge and repeat
Install pallet and spring. Repeat
As you hit an undesired result, stop and find the fault at that subassembly. Then repeat.
As you point out, it could be anything and you need a procedure that allows you to find the fault.