Crazy MPH

For some reason when i start my car after its been off for like 6 hours the MPH stay at 0 when i am driving and it has a hard shift from 1st,2, once it gets into 3rd gear the MPH go to Normal and the shifting is fine.....Does any1 know why this is happening? i hate it cause i cant tell how fast i am going i can only guess thats its somewhere between 0 and 45mph when this problem happens

i have a 99 Malibu has any1 else had thios problem?

its one of your sensors that has failed

how do i replace a sensor?
is it hard to replace one?
Is it bad to drive with a bad sensor?