Last weekend while driving I noticed my speedometer to starting to slowly drop even though I wasn't losing speed. Before I knew it, it was sitting at 0mph while I was driving 75. I plugged in my GPS just to get an accurate speed reading until I could take it into the shop. After the vehicle being off for about 30 minutes though, the speedometer returned to normal function. The first time it moved a little slow and glitchy, but it did read the correct speed. Hasn't done it since, but I'm worried it will again. Any thoughts?