Chevrolet Repair: speedometer sticks on 2003 Cavalier, humid weather, head speed

Question
Hi there.

I have another question I'll send in a separate email to not overload you.

Basically, I've had a used 2003 Cavalier for about 3 years now.  Unfortunately from time to time (perhaps if there is a change from humid to non humid weather and vice versa) it happens that the speedometer which shows the speed I'm going sticks and is usually at 30km or so when the car isn't moving.  Basically, I drive and kind of have to calculate in my head speed = speed + 30 to get an accuracy of how fast I'm going.  This typically clears up over a few times that I turn off and turn on the engine over a 24 hour period.  I do a lot of stop and go driving and I first discovered this in a moist rainy drive that was 1 1/2 hours.

Again, it doesn't happen often, but when it does it's annoying.  I really don't want to spend money on this, but why does this happen?

Thanks for your input.

Answer
HI HRIRE, I HAVE SEEN THIS AND THE FIX HAS ALWAYS BEEN A NEW SPEED HOUSING CLUSTER..IN THE DASH. IT IS NOT THE SENSOR BECAUSE IF IT WERE THE SPPED WOULD NOT REGISTER AT ALL...YOU COULD GET A NEW DASH CLUSTER OR USED...IF YOU DO THIS YOU WILL NEED TO GET IT FLASHED FOR YOUR CAR WITH A SCAN TOOL. FLASH MEANS TO RELEARN THE THE COMPUTER ..GOOD LUCK TO YOU AND I HOPE THIS HELPS YOU..TOM