It could be voltage drop Given the load and size of the cable. Exactly what i was going to say - as the load increases (due to the resistance of the glow pugs) the voltage will drop, 10.5v going to the glow plugs may be quite normal, under load, the battery may not be 100%, with a newer, or more powerful battery, there may be less voltage drop.
Posted April 5, 2019. Glow plugs are part of the emissions system on modern diesels and used during DPF regen which is why they cause limp mode. As you say it only occurs under load and not frequently, it doesn't sound like glow plugs to me, sounds more like an air flow issue, EGR or MAF for example.
source: how esey is it to change the glow plugs on my ford changing spark plugs easy.all you need is spark plug socket.torque wrench would be good.not mandatory if you make sure not to over tighten spark plugs strip out thread since they are aluminum.you need a 3/8 ratchet.short and long extension.you need spark plug gauge to set spark gap.the 2.0 or 2.3 engine spark plug gap is 0.041- 0.045.