Hi steve. As Dave mentioned check starter motor connections .they are crimped into the lugs and can work loose .also the injection loom earth on the side of the inlet manifold.
.might be worth running a wire from the fuel pump live to the inside of car and putting a bulb on it to monitor wether it gets power whilst you have the issue.dont neglect to check the pump negative lead either.or replace it to be sure to a new cleaned point nearby.
I have had very similar issues in the past on these injection systems whereby it was caused by either sticky or leaking injectors .(leaking through into the manifold not externally)
Also check that the airflow meter flap is not sticky.it can be reached by removing the big flexi.
Other than that it’s handy that you have the royale.if I remember correctly it’s also a 3 litre of similar age so parts can be exchanged between if needs be for free .
Other things to check that you have more than likely checked already..........air leaks from gaskets ,injector base o rings,servo hose(very common),airflow meter big flexi hose,pressure build up in crankcase due to blocked valve cover mesh/ breather,vacuum build up in fuel tank vents due to blocked breather,split in fuel pressure valve vacuum hose,split in auto gearbox vacuum valve hose(leads to small metal pipe near top back of inlet manifold,incorrect throttle stop screw ,faulty idle air valve or hoses.
A bit to have a go at there steve. Happy hunting mate