I have two backs, one of them had been serviced some time ago. Frame spacing is about 5mm with the serviced one and 2mm with the other back. The frame spacing can be fine tuned by a mechanic but I have no clue how this works. Sometimes weird spacing can occur if the film does sit too loose on the insert, but this leds to increased spacings in general. (But please do not try to solve your problem this way ;-) )
Small frame spaces occure sometimes when the advance crank is operated too fast. Just try to move it really slow when advancing.
I really hope this would help you...
Congratulations to this great camera anyhow! Feel free to ask if you should have more questions...
Best regards, Benjamin