Automatic transmission slipping in gear under low load

The only thing you can really look at is the transmission fluid, unless you want to look at the transmission itself. Transmission fluid is part lubricant, part hydraulic fluid, the engine spinning pumps the fluid around the transmission making the whole thing work. More engine power pumps more fluid, ordinarily idle RPM is enough for smooth operation but if it isn't working right at low RPM the following problems are things you can check for:

• Low fluid: if you have a slow leak somewhere you can lose transmission oil until the level is too low, and then it will slip at lower RPM
• Old fluid: over time transmission fluid will break down and become less effective, sometimes being harder to circulate, more engine power will force it round better. Old fluid will usually be black and viscous

So the first thing is to check your fluid using the method in your owners manual, usually that is with the engine running. If it is low add some more and the problem should go away temporarily. If you have a leak the problem will come back eventually, how soon depends on how bad the leak is. You can live with it, checking it often, but be aware that the problem could get worse. If the fluid is old and nasty the only fix it is to change the fluid, which can be done by a home mechanic with the right tools and the capacity to get under the car. The transmission filter would need to be changed too, in fact the filter itself could be the problem if it has gotten gummed up.

If you don't have the tools, time or inclination besides checking the fluid all you can really do is take it to a transmission shop.