Intermittent start

My 1988 Vanden Plas has intermittent start. It does start, but occasionally I have to turn the ignition on and off several times before it will start. Sometimes I have to connect a jumper cable from the negative terminal of the battery directly to the engine to start it. The instrument panel lights and indicates there is enough amperage to start the car. I believe the problem is caused by a poor ground from the battery to the engine.
Where is the ground cable from the battery located on the engine, and how can I access it? Could there be something else causing this?

Sounds suspiciously like lack of fuel pressure in the fuel rail due to a faulty fuel pressure regulator (FPR).
A common fault is that the ‘hold pressure’ in the fuel rail after engine shut down is not being maintained to facilitate the next start due to fuel draining back to the tank through a failing FPR (or a leaking check valve (NRV) in the fuel pump outlet)

What then happens is that the necessary fuel pressure in the rail (about 40 psi) is only achieved after frequently switching the ignition ON/OFF/ON etc about six times to take advantage of the ‘priming burst’ from the fuel pump each and every time you turn on the ignition.
You can check this by having someone immediately (and temporarily) clamping the flexible fuel return line downstream of the FPR as the engine is switched off. After a while, restart the engine and as soon as it fires remove the fuel line clamp. If it then starts first time, lack of fuel pressure is undoubtedly the problem due to a faulty FPR.
If it still doesn’t fire first time then the problem may be the fuel pump NRV and the only way to check that is to replace the NRV, item 2 in this picture -

Wouldn’t a faulty fuel pressure regulator cause the car not to start each and every time it is shut down? Sometimes it starts on first turn, and other times I have to ignition on and off several times.
Also, is the negative cable of the battery connected only to the firewall, or further along to the engine. I suspect there might be corrosion causing poor contact.

Also, wouldn’t the engine crank but not start if the NRV was faulty? I am not getting the starter to turn.

Ahhh! The subtle difference between no crank and no start!

IIRC, the engine-to-chasis ground is on the bell-housing under one of the starter motor attachment bolts.

Have you checked/replaced the starter relay?
During attempted cranking, have you tried ‘jiggling’ the gear selector in ‘Park’ or tried it in "Neutral’ in case the problem is in the starter inhibit function of the gear selector?

Thanks Brian. I will search for the ground on the bell-housing and make sure it is making good contact. If this does not resolve the issue I will check/replace the starter relay.

Couldn’t I add a ground by attaching a cable to the negative battery terminal and connect it somewhere on the engine? This would be much simpler that trying to access the starter motor, which would entail draining the transmission and removing the fluid filler tube according to Hanes Manual.
Thanks for all your help.

That’s not how I remember it to gain access to the ground bolt, but it has been a long time. Perhaps someone with more recent experience of cleaning up that ground could chip in here?

SOLVED. I changed the starter relay, and installed an additional ground strap. I think the starter relay was the problem.

Well, the problem is back. Went to start it yesterday and the ignition would not crank the motor. After several turns on the key I heard the starter relay engage and the motor finally cranked. Now I am thinking that it is a bad ignition switch. Maybe a short is causing the ignition switch to not engage the starter relay. I will update my progress after installing a new ignition switch.