Yes ..I think that is the problem...My old relay for the series plugs could not hold the new current levels so I went with the new relay for the extra current capacity....You could buy a couple of those old Ford type fender-mount starter relays for a few bucks each at any parts store and use them instead...2 plugs for each relay...2 relays distributes the load a bit..
Just use an automotive momentary push button switch to energise them both for about 10 seconds and it will start right up....The difference on my 300D was amazing....
Plus you can hold the switch in for a bit if you want to glow extra after starting to help the warm up.....Total manual control....great....
I am thinking of doing this to my 240D just for the extra control....
Since you already have the plugs...you just need the relays and push button switch...about 20 bucks or so...
I know those slow start problems are terrible....they really tested my internal rage levels....

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1978 Yellow 300D (The Mustard Toad)
1980 Blue 240D (The Iron Toad)
1989 Grey Mitsu.4WD Mighty Max Pickup (Needs a Diesel transplant bad)

(Open the pod bay doors HAL)