Error 75 File/Path access error, sometimes Error 1004

I have been making a large-ish userform project over the last 2 weeks. Something is causing a fatal error about every 15 minutes. Most of the time the code executes smoothly, but SOMETIMES when I execute a procedure or save the workbook, I get either Error 75 File/Path access error or Error 1004 Object not Found. When debugging is possible, userform.show is highlighted. But if the error comes on the save event (autosave included), excel just crashes.

I have tried to get in the habit of saving the workbook every time I do anything, but this is really getting more frequent and it's getting under my skin. I've found several other people having similar issues on other forums, but no real answers. I have read that MS Query may have a memory leak or that perhaps it could be caused by how I am loading the combo boxes in the listform. But I would guess that such an error would throw EVERY time, not only some of the time.

So far I have debugged my code and copied my entire project into a new workbook, a version of which is attached. Please open the workbook and fool with it - open the userforms and change the combo boxes, etc for a while with the VBA editor open (while you're at it feel free to look over my code for what might be causing the error..) It will crash eventually.

PS - several places I have put pieces of ADO code. for obvious reasons I have changed the server path to \\server\quotes1.accdb - so i have also attached an empty file called quotes1.accdb, which you can save to your desktop and modify those references to make the whole code work for you.