Flopping over is too much castor.
Slow steering until it comes off center, whereas it has a positive feedback(oversteer?)

Very annoying.

cliff

I had something like that on my KLX 250. I would go into a turn and it would want to fall over. Turns out the hole for the steering lock post went all the way through the steering head, allowing water to get in, the bearings were rusted.

 OK I did what Tinkicker suggested and the chassis is straight within 1/16"
I also took the wheels off and installed them on a trueing stand with a dial indicated and they are true within 1mm
I installed All Balls tapered steering stem headset and swapped the taller front over from the other bike, fork angle increased to 32 degrees.
but no luck, its still wobbly with a low speed weave, it doesnt wanna go straight, all over the lane, my other DT's arent like this.
I cant follow cars in slow traffic without weaving. above 40mph its more stable
Frustrating! Feel like ive tried everything. Dont know where to go from here...

So back to the offset of the yokes-- triples clamps in modern language. Are they originals for this bike. I'm out of ideas, stupid ideas or otherwise.

I think you said you have equal PSI in the front and rear tires. Shouldn't the rear be 4-5 PSI higher than the front? Sounds frustrating, and dangerous.

Pump both tires up to 30psi & head down the road again.

The triple clamps are the original ones, its so strange how it still weaves after I changed so much on te front end! Ill try a much higher PSI or different tires maybe next i think??
Its not a super dangerous weave that makes me lose control and its ok at higher speeds, it just makes me not feel confident on the bike in traffic n trailwork, its hard to explain, its like riding on an edge.

I set the headset preload so that the bars would just barely flop on their own to the side with the bike in the air.

I would swap the front tire/wheel assembly on both machines. Maybe a defective tire or distorted tube?

You posted that one head was 29 degrees and the other 31. Did you ever resolve that difference?

Good luck!

Could it be a bad bead on the front or rear tire?

Might the frame be cracked, maybe by the steering head that isn’t visible until a load  is applied?