Homeplug AP Weirdness

I'm having some strange issues with my Solwise MediaXtream homeplug units.

The network looks something like:

O2 Wireless box -> Homeplug1 & media streamer.
Homeplug2 -> media streamer
Homeplug3 -> gig switch1
Switch1 -> TV, Sky, NAS, media streamer, gig switch2
Switch2 -> Printer, Windows PC, Raspberry Pi, WiFi Access Point

Switch2 and it's associated kit is in the basement where the O2 box wireless doesn't reach. I only want WiFi for my phone & tablet. However, when the AP is plugged in the network connection degrades massively - but only on traffic that is going though the homeplugs. Traffic from the PC tot he NAS (via the 2 switches) is completely unaffected. It's so slow that web pages timeout (including that of the O2 box interface).

Initially I thought it had something to do with the AP, so I've tried a different brand and the same issue occurs, so I'm thinking it's the homeplugs. Powering them off and on fixes the issue as long as the AP is not on. Current unit is an Edimax, previous was a Buffalo.

same problem with solwise ar7400 homeplugs [av500]

they need to be unplugged and plugged back in to start working again

devolo av200 were rock solid bgut only getting 7MB/s with av500 i am getting around 21 MB/s which is pretty impressive

On the Solwise forums, I was advised to change the MTU on the Wireless Access Point. Unfortunately, the Edimax does not have a configurable MTU. So, I'm binning the homeplugs and running Ethernet from the router into the crawlspace under the house to the basement and connecting the switch to the router. Homeplug 2 will be made redundant by drilling a hole through the floor into the basement. I'm going to use the wall grommets that Sky use to seal up the holes.