Sorry David but you have it back to front - the longer the points are closed the more time you have drawing current through the coil - closed points provide the chassis earth for one end of the coil thus taking current through the low tension winding - when the point open the change in current in the low tension coil induces a voltage and current in the high tension coil. there may be a problem with the condenser but I thought Frazer had changed it at some point.
Check the current that is being drawn through the coil with the points closed and see if it is as per spec on the new coil- the erratic behaviour will be caused by the overheating so cure that and you should be ok.
A thought - what voltage are you getting on the "12v" end of coil - is there a problem with alternator overcharging?