There are many kinds of MP4s. Some use variable rather than constant frame rate. Some use the H.264 codec while others use the H.265 codec. And there are a variety of resolutions and frame rates too. So it kind of depends on what you're working with (and, of course the specs of your computer).

Open one of your MP4s in the free download MediaInfo. Set MediaInfo's View menu to Text and then copy and paste the report to this forum. Once we know what your MP4 is made of we'll better be able to help you out.

Not saying that it is anything to do with the problem but it's worth bearing in mind, this month's Windows updates just 'landed' here last night.

Hi StevenG,

I'm haveing a similar problem trying to import an MP4 from a GoPro into PD15. My MediaInfo data is below. Any help will be greatly appriciated!

Format : MPEG-4
Format profile : JVT
Codec ID : avc1 (avc1)
File size : 706 MiB
Duration : 2 min 57 s
Overall bit rate mode : Variable
Overall bit rate : 33.4 Mb/s
Encoded date : UTC 2018-08-16 07:32:25
Tagged date : UTC 2018-08-16 15:32:20
Writing library : Apple QuickTime
xyz : +45.2884-122.6737/
Video
ID : 1
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L4.1
Format settings : CABAC / 2 Ref Frames
Format settings, CABAC : Yes
Format settings, RefFrames : 2 frames
Format settings, GOP : M=1, N=15
Codec ID : avc1
Codec ID/Info : Advanced Video Coding
Duration : 2 min 57 s
Bit rate : 33.2 Mb/s
Width : 1 920 pixels
Clean aperture width : 1 920 pixels
Height : 1 080 pixels
Clean aperture height : 1 080 pixels
Display aspect ratio : 16:9
Clean aperture display aspect ratio : 16:9
Frame rate mode : Constant
Frame rate : 23.976 (24000/1001) FPS
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Bits/(Pixel*Frame) : 0.668
Stream size : 701 MiB (99%)
Title : Core Media Video
Language : English
Encoded date : UTC 2018-08-16 07:32:25
Tagged date : UTC 2018-08-16 15:32:20
Color range : Limited
Color primaries : BT.709
Transfer characteristics : BT.709
Matrix coefficients : BT.709
Audio
ID : 2
Format : mp4a
Codec ID : mp4a
Duration : 2 min 57 s
Bit rate mode : Variable
Bit rate : 192 kb/s
Channel(s) : 2 channels
Sampling rate : 48.0 kHz
Stream size : 4.00 MiB (1%)
Language : English
Encoded date : UTC 2018-08-16 07:32:25
Tagged date : UTC 2018-08-16 15:32:20
Other #1
ID : 3
Type : Time code
Format : QuickTime TC
Duration : 2 min 57 s
Time code of first frame : 07:51:37:05
Time code, striped : Yes
Language : English
Default : No
Encoded date : UTC 2018-08-16 07:32:25
Tagged date : UTC 2018-08-16 15:32:20
Other #2
Type : meta
Duration : 2 min 56 s
Bit rate mode : VBR