RPM's increase, speed decreases

1999 Chevrolet Tahoe LS 5.7 with 60,000. Purchased vehicle new.
While driving at highway speeds, occasionally will notice that rpm's increase, while speed goes down. 'Playing' with the accelerator will rectify on most occasions. Also notice that the same will happen during 'hard' acceleration, when attempting to pass or when merging onto a highway.
Local mechanic is not having luck in fixing the problem. Vehicle has not had a major tuneup, tho' mechanic claims that would not be the issue. Otherwise runs fine, and fuel consumption is normal.