There is a software upgrade to fix the car from going into neutral at a stop. Your problem could be related. It's called the stop-neutral fix. Don't know why Volvo tried this terrible experiment for this particular year but so many complained about it that they came up with a fix. I'm not sure if it's free or not but might be worth asking if it's a free recall upgrade. So...long story short, go to the dealer and demand the software fix.

The other possibility is that indeed your car is having trouble with the ETM (electronic throttle module). This for sure is a free fix at the dealer...they now warranty that part for 10 years or 200K miles. Either way, it's a trip to the dealer for you. Your car doesn't have enough miles for major transmission issues, but a tranny flush would be a good thing to do NOW to prevent such issues in the future.
--
1998 S70 T5 Emarald Green Metallic (125K), 2004 V70 2.5T Ruby Red (45K), Previous Owner of Black '94 850 Turbo Wagon (Sold at 140K and miss her). My cars have been running so well lately they've got me worried.