MacBook Pro 16 (Intel 2019) SSD hangs when transferring to external NVME SSD

Hi,

I recently bought a new external NVME SSD and noticed when transferring files from my Mac to the drive the transfer hangs after a few gigs (4-6). This pauses for a while then starts again before repeating that cycle until the transfer is complete. I initially thought it was the drive so requested a replacement with Amazon. Same issue with the second one. Tried different cables too with no luck. I did a bit of investigation and reviewing of the transfer speeds in Activity Monitor and it looks like the transfer from the internal SSD maxes out at ~1000mb/s before immediately dropping to 0mb/s, hanging before then starting again. So essentially the max speeds are not being sustained. This cycle repeats itself until the transfer is complete.

I tried my wife's M1 MacBook Air and no issues, sustained transfers of ~1000gb with the exact same set of Files. So the issue is my computer.

I've reformatted the drive to multiple different formats (exfat, Mac OS extended etc) all demonstrating the same issue.

The SSD is a 1TB SanDisk Extreme Pro NVME USB-C SSD. I have the previous non pro version of the SSD for a couple years now and it's been great, but max transfer is ~500mb/s so was looking for upgrade. But with the hanging it on the new NVME drive the older one is actually faster. I'm also running a fairly maxed out MacBook Pro 16 inch, intel model bought April 2020. Core I9, 32GB Ram etc. now on Monterey, but I saw this same issue with Big Sur too. Not sure if this would have happened on the older MacOS's as I didn't have the drive at the time.

From my review it really looks like my internal SSD is the issue but looking to see if there is possibly a simple fix, or if this is a known issue. Thanks for any advice.