My Ft100 is only about 2.5 years or so old, and it hasn't had that much use on HF as I operate it mainly on 2M in my old LTD.
I always had problems on HF with varying output right from new, others would tell me my signal varied from S2 to S9 whilst mobile, I replaced the antenna, then the base, and then the coax several times, the problem would come and go intermittantly nothing seemed to cure the fault, I even did other mods available on the internet, which Yaesu them selves made to other FT100's or so the article said, I thought maybe its taking off somehow.
Then one night while operating 6m mobile the HF finals went out I only had 3 watts out with about 12 or so amps of dc input, and the little rig was running red hot, I quickly shut down for fear of damage.
As I replaced them I saw that the drain of the final transistors were connected to the RF output transformer with a minimal amount of solder, this was brought about by the way Yaesu mounted the RF transformer primary into slots cut into the main board in front of the drain connections.
This leaves a gap of up to 1/8" and they had bridged the gap with solder which in my case was a very poor job, and the cause of the varying output in my rig.
The Fet's are fed independantly with 12 volts via a balanced choke wound around a ferrite ring and connected directly to the drains, the consequences are obvious, the power would vary as the soldered connection to the output transformer jiggled around, the finals became tired of working into a load/ no load situation and gave up the ghost!
When I replaced them I cut some 10 thou copper shim into 2 strips 3/8" long by 1/4" wide tinned them both sides, bent them in the middle and made sure I soldered them solidly to both the drain and across the gap onto the transformer primary, hopefully this should do make sure it doesn't happen again.