Probably because you don't have the load of the lambda? You state that it's only when you apply a heavy electrical load the issue arises, therefore to me it's probable this is voltage drop.

Cosworth's suffer a lot of electrical issues now, which is age related to the wiring.

One common cause of poor voltage is resistance in the fusible links (for which there are two) in the fusebox supply.

All you need to do to verify, is measure the voltage to see if the lambda is getting the correct voltage.

Martin