Originally Posted by Granpa

The most common cause for what you're experiencing is a bad or dirty thrust bearing. The thrust bearing goes bad more often than most people think. Unfortunately there is no good way to check it.

The other cause could be your cone or Belleville washers have become flattened and no longer apply pressure as they did. Check them against some new ones. They need to be replaced from time to time. They are available from McMaster Carr inexpensively.