Ground connections/battery terminals seems like something worth checking for sure. On US market diesels (through 1986) the alternator mounts low on the right side of the motor, near the turbo, and one of its mounting points is direct to metal (no rubber bushing) so our diesels don't use a ground wire for the alt case; however, gasssers here (with rubber isolators on all alt mounting points) do need a wire. Don't know how your later Euro-market diesel is configured, but if it's an all-rubber mounting alt that uses a ground wire, you'll want to particularly check those connections.
What's your charging voltage? I had a 245 a few years back that developed a similar, intermittent issue... traced it to a faulty (aftermarket) brush pack/voltage regulator which was letting alternator output vary in a pulsing cycle, for reasons I don't understand. Alternator was an aftermarket refurb with an unknown-brand regulator in it... put a Bosch part in and all was well. You say yours was overhauled a few years ago -- it wouldn't be too soon for something on it to fail, especially if that part is sub-OE quality.
You'd be able to test for any of this with a voltmeter on the battery terminals while idling. Voltmeter across the two batt posts will show charging voltage and any fluctuation. Meter (set on VDC) across alternator case to battery negative terminal, or to car body, or from any other place you suspect possible/intermittent voltage drop (e.g. from battery negative terminal to post, car body to negative post or to alternator case, will show any drop present as a positive or negative reading... etc. If it's a ground integrity problem, you'll pinpoint it this way. Or if it's GP's cycling (somehow?) you can test for voltage at the GP buss when the symptom is present... Or if it's in the alternator, you'll find grounds all OK but fluctuating charge V whenever the problem is occurring.
As long as you can manage to get it to present the symptoms while testing, you should be able to put your finger on whatever's causing this with a bit of testing and without having to try any parts in it.
