Hard shifting at low RPM's

That sounds like a worn pump or a failing seal on the solenoid valve that controls the line pressure, at low RPM you have low control pressures and the solenoid sticks until the pressure hits a point where it suddenly releases. Another possible cause for this is a bad engine mount, this was one of the first vehicles to use "smart mounts" when they start to fail they can mess up the shifting because the signal they send out will tell the ECM that there is a problem and not to let the transmission shift. The only way to know if the second item is the cause is to use a scan tool and see if the mount is showing any error codes. They will not trigger the CEL but will show up in the ECM.

So you bring up something interesting. The "smart mounts", I read online on two Nissan forums it is best to unplug all the mounts because of a high fail rate and they have a habit of damaging the ECU when they go out. Could the problem be linked to them being disconnected and will it damage the transmission?

It could be if the wiring was shorted or corrosion caused an issue. The hard shifting could cause damage but it depends on what is causing it.

So I can plug the smart mounts back in to see if that helps and if so, I can probably just replace them. If not have the solenoid on the pressure line check? Thanks for your help.

I would have a scan tool put on it and look for any codes in the transmission first, it can have codes that will not turn the CEL on. Then pressure test the transmission. It could be that the internals are just starting to wear out.