1JZ MX83 tacho issues

[mamba]

I have a 1J in my MX83 and recently my tacho developed a strange symptom. Every now and then the rpm needle would dip to zero and then back or even just sit very low, like 100rpm or something, while the car is on and idling. It seems fine when I drive around.
I also keep getting an intermittent engine light, code 13. It doesn't always come on, only on idle ocasionally. the car still runs and responds perfectly.
What I would like to know is, is code 13 and the tacho responding strangely related? What actually sends the rpm signal? Is there a sensor somewhere that needs to be replaced on the engine or gbox somewhere?
I have already changed my ecu caps, did nothing, also I'm not using a tacho booster, I'm using a jzx81 analogue cluster.
Any help appriciated with the starnge tacho readings and especially a code 13 fix.

[mamba]

Anyone got anything??