Engine surge

my 1990 Nissan 300zx with 136,000 miles often surges at highway speeds for several seconds then drops back down to normal. the surge is usually 1,ooo rpm. does not happen if not in overdrive, and will not happen unless having been driving for 15 minutes or so. also seems to occur more on hot days (80 degree +) than on cool days. mechanic has looked at this several times but not yet able to determine why.

Does it accelerate when the surge happens? Could it be the transmission slipping?

no noticable change in speed, just the surge