Lonnie, from what you descibe, it sounds like the main clutch is slipping. Is there some smoke or foul odor coming from underneath the clutch housing (right behind the engine) ? Does the housing get hot when this happens? Do you have a PTO-driven attachment such as a bush-hog or grinder that you could hook up and see if the attachment continues to run under load?
I bought a Series IV last December and it was sometimes a bear to get into gear. (But not always) What happened over the years is that some hydraulic fluid had leaked past the seal(s) in the hollow hydraulic pump shaft and soaked the clutch plate and made a huge gooey mess.
The hollow shaft was used on the Series IV, not the earlier D17's.
If this is the case, the clutch plate and/or pressure plate will probably have to be installed. This is also the time to replace the seals and bushing inside that hollow shaft.