Check the tension against a good wheel. Start at the valve and turn each nipple half a turn clockwise (as if you're looking down from the nipple towards the hub) and immediately an eighth of a turn back (this takes the twist out of the spoke). Work your way all the way round the wheel and then repeat if it's still not tensioned enough.
I would then pre stress what you've done by grabbing the spokes in handfuls of four (i.e. 2 from each side) and give a good squeeze. When you're done with all that, do your normal Truing adjustments.
I got a set of open pro/105 wheels cheap from
eBay that were severely under tensioned and kept going out of true almost every ride. I went through this process and they have been perfect ever since.
By the way, I'm sure you're aware but 100kg+ is a lot for a 20 spoke wheel. You're risking a failure.
Cheers,
malty