radman
i take it this prob is on the cosworth.
iv'e just fixed the exact same problem on mine, turned out to be, as i changed them both, either the air flow sensor or the temp sensor.
not the temp sensor to the dash, the other one on the back of the head which goes to the ecu.
bought mine off matt lewis
http://cgi.ebay.co.uk/RS-Cosworth-YB...QQcmdZViewItem
hope this helps