Also, depending on the model of the deck, it will have an edit delay setting in the menu. Adjust this setting if your inderts are consistantly off by the same amount. I believe this is set to a 6 frame delay at the factory. Try adjusting it until the offset is zero frames.
Since you refer to your deck as an older SP model, it’s probably a BVW-70 or BVW-75? There is an edit delay setting on this machine, although you might have to enable the service level menus to get to it. There is a small, fairly well hidden switch to turn on the enhanced menu selection. It is accessable at the lower right the fromt panel(going from memory here), but not easy to get at. Check your manual and it will have a diagram of just how to get at the switch. Try changing the setting and see if it makes a difference.
Just a few random engineering-type thoughts…
Tom