This car drove great until last week. It stalled out at 35mph. The car started after 5 or 6 tries. After driving about a block or two I noticed that the speedometer stayed at "0" and the trans hesitated once in a while. After getting home I took the car out again after about one hour. It drove great. My speedometer was back and the car shifted good. I went to a store and spent 20 minutes shopping. Got back in the car and it drove lousy. It even stalled out again at a stop light, no speedometer and trans hesitated. My son is a mechanic. He took the car to work and hooked up the computer diagnosing unit. He got a reading to replace the speed sensor which he did. No luck! The speedometer still is at "0", the trans hesitates and shifts erratically. But sometimes the car rides great for a while, I get the speedometer back with no problems. My son hooked up the diagnosing unit again and couldn't get a code. What's up?