1998 Infiniti qx4 rpm surge

What is your question?

My 1998 Infiniti QX4 has about 160000 miles on it. When driving at about 70 mph for a sustained period, maybe an hour or more, sometimes the rpms jump up about 1000 rpm for a few seconds then drops down. This is very intermittent. This is not when going up an incline, this is on flat level ground. I wonder if the overdrive is kicking in and out. I've taken it to three Infiniti dealers and they have no idea.

What makes this problem better or worse?

Problem occurs after sustained running, maybe an hour or more.

How long has your 1998 INFINITI QX4 had this problem?

About 10 years