Car : 94 S420 with about 80,000 km.
Problem : Car starts easily while cold (ie sitting overnight). When the engine is hot, it is easy to restart (ie restart within about 5 minutes of shutting off the engine). When the engine is warm (ie. sitting for about 30 minutes) it takes about 10 to 12 seconds to restart.
Fixes thus far : replaced fuel pressure regulator (at the fuel rail) and replaced check valve at the fuel pump under the rear of the car. There was a bit of rust on the old check valve.
What's the next thing to check?