A common ALC fault in the X59-1100-00 DC-DC converter board causes TX output power to cycle up and down in regular cycles due to alc voltage variation. This is dependent on the power supply voltage and will show up usually under 13.5v or less.
Instead of replacing the DC-DC converter board replace R2 and R3, 22kohm SMD resistors with a lesser value. I used 15kohm SMD resistors with great results ;ie no power dropoff symptoms down to 9v supply tested. Have heard of 18Kohm values used with good results too.
Another problem i had with one of my TS850's: when switching the Rig ON it came in TX mode sometimes. This was VERY intermittent and sometimes it switched ON ok and then after 1, 5 or even 20 minutes it did go in TX mode. After a long investigation i discovered that on the CARRIER UNIT (X50-3140-00) an electrolytic capacitor was LEAKING! After replacing this capacitor (C1 47 uF SMD device) the problem was gone!
Be aware for LEAKING CAPACITORS. Also Jurgen PE1LWT has bad experience with this and almost destroyed the PCB.