Hi Tim,
now I am back from work to test your tips.
I've changed the delay between the pings many times before. This had no effect.
I've changed the "MAX_SENSOR_DELAY" also, but no effect 
That's why I took a look at the trigger signal with the scope. With the NewPing-Lib and the simple sketch I see most of the time a constant high level at the trigger-pin!
If I use the method "trigger pulse and evaluation by "pulseIn (echo, HIGH)" without NewPing, I get the trigger and echo impulses that I expect. Any ideas what could be wrong?
Regards
Martin