Here's an interesting post from the TS590 list, regarding my problem.

May just have to grab that TS480 power cable from out in the RV.

Tom Behler: KB8TYJ

----- Original Message ----- 
From: "Tom" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, December 10, 2012 9:38 PM
Subject: Re: [TS-590] TS590 Output Power


My TS-590 was doing the same thing - I sent it back to Kenwood and was told 
rig was putting out solid 100 watts output yet all my tests here I was only 
getting 70 watts.

Come to find out I had a bad power cable. I tried another power cable from 
the Yaesu FT-950 and the my TS-590 now put out the expected 100 watts 
instead of 70.

If you search for my original post you can see my findings like you 
explained

Tom AB8RL