97 slipping into neutral at stop light??

Hello everyone,
This is Ronika and my brother just sold (more like gave ;) me his pristine 97 gp w/133,000 miles on it..Its in great shape. he gets a new car every 7 years and this beauty is now mine...He has done excellent maintenance and service on this car. Everything is great except .....When its been running for awhile when I'm at a stoplight w/ my foot on the brake, when I go to accelerate I thought I accidentaly shoved it into neutral because it just revved. I put it into park, then neutral , then drive... No problem. This has happened twice in two weeks. He says it has happend to him at stop lights before..only at a stop..The trans seems to shift ok through the rest of the gears .He had the trans flushed when it was newer and the fluid (dex) looks clean as does all his fluids .
Anyone have any idea what this might mean or what I should look out for or do??
I have already learned from reading this forum to get another gas cap because the ses light was coming on( it worked). My younger brother swears by duralube for his older car..Does anyone recommend red duralube??? Thanks for reading.