Spark not right.. V2.2, easyVR, VB921

[rs2000]

I'm using easyVR and a VB921 to fire a coil. The distributor supplies the VR signal.

Everything works as expected until i hook up the VB921 to the coil -. If i crank the car, the injectors go nuts buzzing and clicking on and off. This goes on for about 30 seconds and usually stops. During this time the rpm in megatune flicks around, as does the pulswidth. The coil also fires while all this is happening.

If it starts going nuts it can be stopped by either disconnecting the VB921 to coil -, or by disconnecting the output of the easyVR circuit.

I have had this ms and VB921 working using a hall effect distributor so i'm fairly sure its not the issue. I'm using msns mode (in megatune) with LED 17 as the spark output (set as Spark Output A).

Code i'm using is keithmac's idle code, which is based on 029L. The board is a V2.2.

Does anyone have any suggestions of what to look for? I've checked over the wiring and it all seems ok. It feels like some sort of feedback, but i'm not sure what to check..

thanks

andrew