Digidash & Speedsix standard ECU

[SkyUK]

I have a 2003 speedsix engine from a Tuscan in my kit car with the standard ECU. Sussex TVR and I have been unable to get the Digidash 2 - Lite rev counter working accurately with the counter jumping +/-1500 from any central point and can reach the 2 extremes in the same second. This make the rev counter and the change gear lights useless.

Does anyone have any suggestions as to how I could cure this or what to look for? I'm stumped. Thanks