My service light comes on then my trans slams into gear when it shifts... Then, quick as it starts it stops... Light goes out and it shifts just fine... What gives?

I had the same problem. My Mechanic hooked up the checker thing and said ti was an oxygen sensor. I said OK but was skeptical and took it to my transmission place, They checked the transmission and said there is nothing at all wrong with the transmission it is an oxygen sensor making the car give too much or too little gas causing the transmission to shift hard. Went back to my mechanic, the sensor was $140 and he charged me $40 to put it in. I drove it over 600 miles, very very hard stop and go miles before I came here to post that the problem is fixed. So if you have this problem; mine is a 2002 L300 V6 but its basically the same for all L300 I hear, if the reading is oxygen sensor is bad replace that and it should fix the hard shifting problem.