gearbox fault.....?

i have noticed over the last few weeks that when driving the car (4.6 auto ) that when driving along at 45mph the gearbox jerks and doesnt go into top gear. if i speed up slowly the car will go upto 50mph before changing into top gear. I know the car has a system as to how the gears are changed to how you drive it, and yes i do floor it from time to time! but is this standard for it to change at 50mph??? i'm sure it used to be 45mph.....
now i will be checking and changing the gearbox oil so this might sort out the slow changing of top gear..... i also dont have any oil leaks.? saying that i havnt checked under the car but know the ground where i park the car is dry...