I'd guess the problem is with the pallet fork or balance assy. A tiny bit of micro fibre fluff on the pallet jewels, or dirt in the diashocks maybe.
The mainspring was fine when I cleaned it and put it back together. However, I'm betting it's related to the diashock jewels. I was dreading that . . . heheWind the mainspring by screwdriver on the screw and feel for any jumps or judders which could mean the spring is broken and therefore not holding much power. Second as feca says clean both diashock jewels its one of those jobs we hate because the bits fly everywhere but its amazing the difference it can make.
I made sure to avoid lubricating the pivots but I did lubricate the jewels as directed.Dont lub the pallet fork.
This might be it. I forgot to mention that it's almost difficult to set because the hands move so freely during setting. Thanks for the advice!Maybe slipping cannon pinion? Super easy moving of the hands when setting the time ( no drag) and not enough "torque" to change the date at midnight is usually a cannon pinion that needs to be tightened.
for a bit extra the 'nr 1900' model has a colour high resolution display, other than that it's about the same machine as the 1000 ..Should be relatively easy to repair
You might want to invest in one of those cheap Chinese "Multifunction nr 1000" timegraphers. I've got one and it's working perfectly