I have 3 Devolo 500 AV units. One is right next to the VDSL master socket, plugged into a power socket. FTTC has been running at 60/20 for the past 9 months with no drop outs or change in sync (interleaving etc).
Could possibly be down build quality of the units, I suppose. I don't know enough to say, or know anything about Solwise compared to Devolo. I do notice some high pitch sound emanating from the Devolo unit when a high amount of traffic is passing through it, but as I said, I see no detrimental affect on the VDSL sync speed.
One thing I do have is a shielded RJ11 cable coming from the master socket into the BT VDSL modem. But I do wonder if that really helps at all considering the normal unshielded phone cable coming into the socket is still in close proximity to the Devolo unit. Only reason I have a shielded RJ11 is because I just like to do things "right", where at all possible.
Not sure what else to suggest other than swap to an alternative brand (if you really feel it is the units causing the problems) or downgrade back to 40/10 (which I do plan to do once my contract is up) which will not push the line sync as high, making it more stable.