1988 740 GLE Auto tranny - won't shift into high gear.

I am working on my daughter's Volvo 1988 740 GLE (Auto) and need advice. The tranny will not shift into high gear about 50% of the time. At about 110 kph (70mph), it is rev'g about 4000. It just started doing this in the past few weeks and I finally got around to looking into it. The tranny fluid has not been changed in a few years; but the level is fine and the colour looks good on the stick. I have seen some comments about kickdown cable adjustment, and I hope that this is the issue. There are only two cables on the aluminum drum for this unit. On the right side (driver's right) there is a linkage where other years might have had a third cable. Help!