Rätzelhafte data rates

I never imagined the Powerline brought to a PC via LAN to be wired to the internet without the whole apartment and to the WLAN to enhance signal. Setting up the system works quite well. I have a Fritz box 3390. Near the router plugged the adapter 510 and the 540 igendwo adapter. In the beginning there were no problems, at least not noticeably. I've tested from and to the Internet speed and get satisfactory transfer rates. But later left to the data rate for no apparent reason. And only on the PC connected via LAN via the powerline to the router. Sometimes the connection broke off completely and I could no longer access the router except WLAN. A notebook constantly reaching accessed via wireless router on the full transfer speeds.
Then I got a little experimenting. The powerline was first connected via a triple extension socket to the mains. As the Abrüche piled up I have the Powerline plugged directly into the wall socket, there could at some point the data rates by. Then I tried other wall boxes there the same problem, the data rates are very good times and very bad times until canceled. As soon as just another consumer in the socket with off is the data rates are small, even if the consumer is not turned on.
At the Powerline unfortunately you do not get direct information about the data rates and whether the Powerline ever still running. Partially could only be re-establish a connection if you pulled the powerline and inserting again.
Today I'm going to the Powerline send back (after nearly a month in the test). The last tests yesterday full power over the powerline only half well if no changes have been made today,
At least the Powerline 540E could not guarantee soild access via LAN to the router. Incidentally, I also had the support of AVM contacted the first place not konnt help me and secondly towards me seemed unfriendly. But only at the margins.