UEFI headache

Hi all,
I think I'm ruing out of ideas here --- and it may well be that I've missed some easy bits and making my (computer)life miserable for nothing, so I'd appreciate any help!

OK: I've got a Dell XPS 15 9550, which came with Windows pre-installed. I don't really use Windows anyway, so I installed *only* Ubuntu 16.04. I followed advice on line (eg here) and did all the relevant prep (disabled secure boot, changed SATA operation from RAID to AHCI). So, all worked fine and I was able to install it on UEFI. For a few days all worked fine, but then (I *think* this may have to do with a keel update, but I didn't keep track of this, so can't say for sure whether that was responsible for it!!), things started to go really badly. Basically, most of the times, I can't get Ubuntu (which is the only OS) to boot. When I fire up my laptop, after a few seconds I get the Dell Support Assist telling me that there are potential hardware problems. Basically, what happens is that I am not able to get to grub. If I enter the BIOS to modify the settings eg trying to add a boot option (I've seen that you sometimes need to do that), most of the times I get a waing pop-up saying that "File System Not Found!".

I can boot to a USB flash and from there, I've tried to fiddle with the /boot/efi/EFI files/settings. I've tried doing this, which occasionally has worked --- meaning that after rebooting I had some success and managed to get into my Ubuntu. But the next day (or simply after a few hours), when I tried to re-boot again, I was back to square one...

I've also tried this (which, as I understand it --- and I may well not know what I'm doing, at this point!) instead of rebuilding the grub-efi simply copies over the files from the ESP to /boot/efi/EFI. Again, I can remember of a couple of times when this has worked, but not consistently and basically I'm not stuck with a machine that 95% of the times doesn't start up! I know I could try to re-install Ubuntu, but I've done this a couple of times already in the past month or so, and I'd really like to avoid it if I can...
Incidentally, this is the output for fdisk and gdisk --- as far as I can tell, things *are* in order: I do have a EFI partition which has a boot flag (checked in gparted) and the GPT/MBR settings seem OK...

ubuntu@ubuntu:~$ sudo fdisk -l /dev/sdb
Disk /dev/sdb: 931.5 GiB, 1000204886016 bytes, 1953525168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: 4C97C1CF-77C8-4EAD-8249-5868B912E1A2

Device Start End Sectors Size Type
/dev/sdb1 2048 1269759 1267712 619M EFI System
/dev/sdb2 1269760 59863039 58593280 28G Linux filesystem
/dev/sdb3 59863040 63768575 3905536 1.9G Linux swap
/dev/sdb4 63768576 1953521663 1889753088 901.1G Linux filesystem

ubuntu@ubuntu:~$ sudo gdisk -l /dev/sdb
GPT fdisk (gdisk) version 1.0.1

Partition table scan:
MBR: protective
BSD: not present
APM: not present
GPT: present

Found valid GPT with protective MBR; using GPT.
Disk /dev/sdb: 1953525168 sectors, 931.5 GiB
Logical sector size: 512 bytes
Disk identifier (GUID): 4C97C1CF-77C8-4EAD-8249-5868B912E1A2
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 1953525134
Partitions will be aligned on 2048-sector boundaries
Total free space is 5485 sectors (2.7 MiB)

Number Start (sector) End (sector) Size Code Name
1 2048 1269759 619.0 MiB EF00
2 1269760 59863039 27.9 GiB 8300
3 59863040 63768575 1.9 GiB 8200
4 63768576 1953521663 901.1 GiB 8300

Does anybody have any idea here?
Many thanks!
Gianluca