My Amiga 2000 is also setup with a rev 4 (green PCB) G-Force 030 @40Mhz. I use that card for everything as it solves most problems all on its own. The machine sports the original dual floppy drives. I use an external GoTek on occasion in rare circumstances.
I have the G-Force maxed out in RAM, thus no other RAM cards are really needed. Not with what I do, anyway. The accelerator is great, too.
The card powers a long SCSI chain that includes the original 40MB HDD, which the machine boots off of. It also drives an internal SyQuest (used for backups only) and 2GB SCSI2SD HDD which is where I install all of my software. I use the card's external SCSI port for a Zip Drive, too.
The A2K also has a megaChip upgrade for 2mb chip RAM.
At the end of the day, hats off to my old A2K's PSU!
That all being said, even though the machine can obviously have a lot more cards inserted I don't really need any. So there's just a lot of space in there. I've pondered adding a Serial Port card to add more ports, but I've procrastinated.
The G-Force kinda hits all the right buttons for that machine. I don't use the A2K for storage of other cards not in use, fwiw.
Now, mine is running OS 1.3 and not anything recent. And it's as solid as a rock.
After all of your tests you've done thus far, do you think your issue is software-based or hardware at this point?