It seems that since Saturday at about 16:00 UCT, the issue with our hosting server has been solved. After several contacts and complaints, my measurements with Pingdom made enough pressure to move the support there to help us. Our site has now been moved to another server. Here for example the measured downtimes since Saturday:
Not one minute down since the change! The yellow marker is the time of move of our site. Also the response times have significantly decreased:
The average response time (measured from servers all over the world) is 0.624 sec for Sunday, with is very good. I hope we will stay stable from now one near these values. Let's see what happens on Monday. I continue to monitor these values and will get alerted, if we have again downtimes.