Very flat partial

[Donn]

but why would that make just ONE partial so flat

You've reported at least two partials with flat notes, the way I understand it. 3rd partial starting at G, and 4th partial starting at C and adding valves 1 and 3 to get G.

[jeopardymaster]

Re blobs of solder, gunk, spurs - I'm no physicist, but I'm pretty sure that the closer an aberration is to the mouthpiece, the more likely it is to affect intonation, and the less significant it has to be in terms of size. Case in point - my 184 has always had good intonation, but initially I had a VERY flat 8th partial. I had to play middle c with my 5th valve (sharp 2-3 configuration). When I described that issue to a fellow in Pittsburgh named Ted Woehr (spelling?), he offered to look at it. With a borescope he discovered a burr at the water key. He smoothed it out, and voila! - brought that note in line. It might be worthwhile for you to check the insides of the horn for imperfections, especially from the receiver through the valve block to the tuning slide.