Hi, have recently experienced similar issues.
How did you get on repairing?
1 st incidence we was due to oil not coming back out of lift cylinder until we removed the velocity fuse / anti drop valve / safety valve.
Replaced the velocity fuse and truck appears ok.
This is the first time I have come across a velocity fuse that stops the descent / oil 100%. This I believe explains why the platform will no lower at all once the platform is at 100% lift height and the velocity fuse has tripped, (line pressure holding valve shut).
Previously all velocity fuses on forklifts I have seen have a controlled lowering rate once the velocity fuse is tripped / activated.
2nd incidence of wave stuck in the air is happening today, see what results??