After a hard run and lots of driving in hot weather, as I was driving home......I could'nt rev more than 3,000 rpm, no matter what gear i was in. Car could'nt go more than 50 mph. Lasted for about 2 miles. Then, the alternator light came on....and the car started running normally. First time it's happened....any ideas what it could be. As I said, it was a hot day and I'd been driving it hard.