My NL fairlane 98 has started to turn itself off and won't start unless I disconnect the battery for a few minutes. But other times it will not turn off and I have to stall it.
When the car dies no lights come on and revs slowly drop unless I put my foot on the clutch then all power is lost, so no power steering or any of that. The radio will still play so accessories work once it dies but wont start. Ive changed spark plugs, leads, coil, ignition barrel. Any help would be muchly appreciated.

Does the engine still turn over but not start? Checked the distributor?

Check throttle body and ISC. Make sure they are clean and ISC and TP sensors are working okay.