It is common when under the pressure of a tournament, bet or just peer pressure to have your golf swing change a little. Normally we assume we are making mechanical mistakes. This does happen but more often than not your swing has not changed but your tempo has increase in speed. 
This normally happens with a quicker take away and also a faster transition at the top. We often wrongly assume we are making a bad down swing and try to correct that when the problem happened earlier in the swing.
It is important to let your lower body start the down swing before the hands and arms start to apply force. Your lower body has more mass and takes longer to move than your arms so you need to give them a head start.
There are many drills out there you can use to work on this but the key is to understand if you are quick at the top you will probably hit a lot of bad shots. Humming or counting during your swing helps many golfers.
Next time you start hitting bad shots under pressure try to think about your tempo before you jump into mechanical thoughts.
Jim Hartnett, PGA
