Sounds awful but one cheap improvement is to replace the steel nuts on the starter terminals with brass ones. I have neticed a huge variation in the price of Espace starter motors from the same firm, Bolk (Chinese?) seem to be the cheapest whilst Bosch are the dearest at about five times the price of the cheap ones.
What I do know is that the cheap starter motors on the engines used to power 5KW generators normally have moulded commutators (the copper segments are just stuck into plastic) Prolonged cranking can cause the segments to become unstuck!)
Heavy duty commutators are made like wedges of cheese and they are held in place with dovetails.
Most vehicles have a Canadian option, this is a battery with 50% more capacity and a heavy duty starter.
Note that the design of most starter motors is illogical and the same goes for alternators. The problem is how they are earthed. Usually earthing is done at the drive end hence the current has to travel down thin screws or via very dubious connections between the "pot" and the end flanges. This is in fact a common failure on the starters of some well known American motorcycles. In this case the cure is to run a heavy earth braid from one of the starter motor mounting bolts and bond this to the end cap.
Some modern alternators now have an earthing point on the end cap. This is far better than relying on numerous fortuitous joints which may rust. In the case of my Ford Granada 2.8 there were about a dozen fortuilous joints between alternator B- and battery B- and in the damp climate some had rusted. A yard of earth braid transformed the car!
Getting back to the Espace, the starters have a steady bracket at the terminal end. It could well be that when the engine block is very hot and expands, this bracket tries to pull the starter apart and causes bad fortuitous connections (the ones between the end flanges and the central "pot") As to the resistance of the starter solenoid coil, if it was 50 ohms at 20 degrees C it would be about 65 ohms at 100 degrees C and I just cannot believe that this modest increase in rsistance is the cause of the malfunction. Good luck!
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