S280 Gears

[madhibby2000]

My '03 reg S280 is intermittently (getting more frequent) not not changing up the gears. When I start car & put into "drive" there's a shudder then when I move off, the rev counter goes high and its like it is driving in 3rd gear. Usually when I stop, turn off engine and re-start its ok. However, today it wouldn't clear and was continually in this low gear with rev counter extremely high. Any ideas? thanks