Kimmo, great picture demonstrating the right-hand-rule! Works for electromagnetism as well as spoke-tension!!!
On the OP's problem, custom-built wheels aren't a very good profit-area for shop and they tend to not devote sufficient time to do high-quality builds. Unfortunately, a lot of folks, including shop personnel, think that "customizing" a wheel, as in choosing just the hub, spokes and rims that you want, is somehow "better" than stock. But, it's really the build-process and even spoke-tensioning towards the high-end that will generate the quality. Most shops will gladly sell you the parts, but skimp on the labour.
Unfortunately, in this case, it's may be too little, and perhaps even too late. If your wheels have been re-trued numerous times, the nipple-threads may be worn to the point where they will tend to loosen gradually, even
IF brought up to proper tension. If your spokes are loosening regularly to the point where you have to spin the nipples
several turns to build up tension, there's a major problem here. Perhaps, the only solution might be to rebuild with new nipples and get it up to the high-end of the tension-range the 1st time.
Reducing dish as much as possible will give you the strongest wheel possible due to evening out the left-right tension-imbalance:
1. use 135mm OLD spacing on the rear, the bike should already have rear-end at that spacing
2. optimize hub for 135mm OLD. Depending upon the rear-hub you choose, it may have come with 130mm spacing for a road-hub. In which case, the quick & dirty way to adapt it to 135mm spacing is to put 2.5mm spacers on each end of the axle. This does NOTHING to take advantage of the wider spacing (by reducing dish). In order to do it right, one places the extra 5mm all on the left non-drive side. I go an extra step and move an additional 1-3mm of washers to the left to place the smallest cog only 4mm away from the dropout surface. This reduces dish as much as possible and makes for a stronger wheel by minimzing left-right side spoke-tensions.
3. use laterally-stiff rims. For the same weight, box-section rims are stronger laterally than aero rims. They tend to keep spoke-tensions even all around better by resisting lateral bending and staying true longer.
4. use larger tyres at lower-pressures. For your 350lb load, I'd recommend at least 28mm tyres, 32-35mm would be best at 85-90psi. The lower pressure absorbs more road-shock with better damping, thus reducing peak-G forces on the wheel when hitting any kind of road irregularity.