recurring rust problem with Vaillant combi/heating?

I have a Vaillant combi VCW280, 5 years old, with Hudevad radiators. We are going through a recurring cycle of DHW heat exchanger primary side clogging up with quite large rust particles, with the radiators also clogging up. The rad system has now been power flushed and dosed a number of times, but still the sytem appears to be producing rust, whihc the DHE HE eventually clogging up (and producing HW whihc cycles in temperature.) Any ideas on why this is happening, and how it can be stopped, as 5 plumbers now have failed to solve it.