I had exactly the same problems with my 98 ST2 last summer. I ended up replacing the Regulator and fitting an uprated ignition wiring kit from Powerlet (
www.powerletproducts.com), I think you can order them from the UK as well from here (
http://www.blue-job.co.uk/).
It was very simple to fit and the delivery from the USA was very quick. It's worked fine ever since (Fingers and toes crossed, touching lots of wood and things)