I bought my FT-817 shortly after it was launched. It was supplied with a ferrite choke from Yaesu to fix on the DC power lead to solve instability on 440MHz. This worked fine but I have noticed that mine has a tendency to produce full output on 144MHz when switched to SSB and not speaking suggesting PA instability.
Today I did a search on "FT-817 Instability" and came across a post by KC3VO from 2003 describing a modification to stabilise the FT-817 PA. It looked easy to implement as it basically involved swapping over the centre and braid wires of the input coax to the FT-817 PA module. Ten minutes later the mod was done. Testing it out I could not provoke the same reaction on 144MHz. Problem solved!