I have this weird problem that the car suddenly stops running while on the road. It has happened 3 times in the past 14 months. Every time it happened, my speed was less than 10 mph in slow traffic. And it was after I had driven over 15 minutes on the highway without any signs of trouble. The check engine light and low engine oil light would come on while the stall happened. After the stall, the car would not start. But after 20 minutes or so, the car would start normally, and it would run without any problem for months.
I have brought the car to a mechanic to diagnose the problem. I was told that it might have been caused by a faulty exhaust valve. The valve was replaced 5 months ago, but the stall happened again earlier this week. About a month ago the starter died and I went to a different shop to replace it. I was told that the cover gasket was leaky but it should not cause noticeable problem. After the starter replacement, I also started to notice that the car would occasionally vibrate abnormally for about 30 seconds after starting. I am not sure whether this is related to the stalling problem.
I cannot tell whether it is an engine problem or an electrical problem that is causing the engine to stall. I would greatly appreciate it if you can point out what tests I should run to pinpoint the problem.