Video Assist Dropping Frames

I have tried using the video assist 5" a number of times with a dozen different approved SD cards and I always get the same result... Dropped frames. I'm recording at DNxHD 45.

Yesterday it went 40 minutes without dropping a frame. Then on the next record it dropped 5 minutes in.

I copied off the data cleared the drive and started another recording expecting it to go 40 minutes, and it dropped at about 7 minutes, so I stopped the recording and started again, 2 minutes later it dropped again.

I've tried every SanDisk card I have they are all Extreme Pro UHS 3 cards from 64GB to 128GB. The BM page claims I can get by with UHS 1 cards.

I've upgraded the firmware and still no joy. Any opinions on how I can make this thing work or are they notoriously droppy?

I have the same problem with dropping frames too. I've tried different SD cards but still the same problem.

Also having problems with frame-dropping on a 5" video assist. just got the 'dropped frames' notification, but the other indications were that recording was still continuing. However the end of a 'critical' recording is just not there.
Using a Sandisk extreme pro 95MBs 64GB card, which has always worked before (have had trouble with other cards). Recording 720 50p from a BMD studio mixer.
We've managed to record hours before - this was quite a shock.
Why would dropped frames stop the recording?