Celica Charging - Dash Power Fail

[1990 GTR]

Last night all the internal power to my 1991 GTR failed - all the guages back to zero and no fans/wipers and the Battery went flat. ( engine kept going and lights still worked until batt died )

got the car home and charged the Battery and the car started fine - everything normal.

after about 10 mins, all the gauges went to zero again and putting a meter on the alternator revealed it was at Battery voltage - not charging at it's usual 14.6v. - switch off and 10 mins later everything normal again !!!!

could this be down to the regulator in the alternater overheating/tripping out and if so, why does the interior power go off??

had the alternater bench tested and cold, it seemed to be fine for the short time they tested it

If cabin power went down, this would remove the 12v sense to the alternator - would that stop it charging.

and on that note, what would cut power to the interior - guages, wipers and heater blower???

any help much appreciated

Nick