All of a sudden today I could not accelerate. Only owner and 135K and first time this has ever happened. Starts fine, runs fine but after driving a few miles could not increase speed. I was on my way to doc's office and 'limped' there and when I drove home it didn't happen again but now I am nervous. Half tank of gas and oil and other fluids are fine and no idiot lights. Looked under hood and nothing obvious to my 'non mechanic's eye'. Looking for advice as to what it might be.