964 misfire help

[adrianc]

I have a 90 964 C2 with a bad bad misfire.

It always starts and runs perfect initially, but after a short time the car starts running so badly its a job to keep it going.
The dme flicks from live to earth continually.

Possibly related???when misfire starts the convertible hood warning lamp flicks on and off....

Checked;
2nd distributer. Fine.
Changed flywheel sensor.
Chenged DME relay.
Changed ecu with known good one,
Removed/bypassed original alarm
Bypassed aftermarket immobiliser.

Checked under fuse box for damage.
Stripped loom in engine bay and through car.
Bypassed battery cutoff switch.
Removed hood ecu.:

What simple thing have we missed?

Thanks.