Hey Roger, yes I understand this problem for sure. The inconsistent contact can cause the spin to vary a lot and can be frustrating. I would take the focus away from doing a good serve over the net and really focus on consistent contact. To do this try to get the ball spinning back to you but just on your side of the table and not over the net. So just trying to contact the ball at the leading edge and with a very light and brushing contact.
Then to progress this to a proper serve you just need to add the forward motion and follow through in the serve once you feel the contact is more consistent.
Secondly I would say try to think about the timing of the serve and getting that consistent too, breaking the serve into 2 parts can help this. So think about the ball throw and backswing as the first part, and then contact and follow through as part 2, this will help more solid timing with the serve. You can even count in your head 1 and 2 as you throw the ball up so you don’t rush and take your time with each serve.
Feel free to send us in a video of you serving and we’ll be happy to analyse it and give you feedback but hope that little drill will help in the meantime 🙂