Hi, all.

Well, I'm happy to report that I have solved the problem of reduced power 
output on my TS590.

The culprit was my DC power cord.

I braved the 25 degrees and snow here, and took a trip out to the RV to 
retrieve the power cord to my TS480.  I exchanged power cords, and voila: 
the TS590 put out a solid and consistent 100 watts, as read by the rig's 
watt meter.  My LDG TW1 gave me a slightly lower reading of 90 watts, but 
I'm not going to worry about the extra ten watts, since that difference 
might be a factor of the lack of accuracy of the LDG.

So, my advice to everyone would be to check your DC power cords, since the 
590 is obviously very sensitive to the slightest problem, and will cut power 
back accordingly.

Now, I guess I'll either need to buy another power cord for the TS480 out in 
the RV, or try to figure out what is wrong with my defective cord.

Thanks to all for allowing this thread to evolve over the last day or so.

73 from Tom Behler: KB8TYJ