Problem getting pc's to actually communicate

My network consist of an AT&T Motorola NVG 510 modem/wireless router, one Dell Windows 8.1 laptop, one Acer Aspire Windows 7 rarely used laptop, another Aspire W7 laptop, that is only used for streaming netfilx, and occasional extra storage. Then I have an Inspiron tower with XP, that is only used for educational DVD's for my 9 y/o, we occasionally do connect to the internet to update the software, or download other games. Then we have 4 Android phones, of course we can't forget my laptop, I multi-boot W8.1, and 3 flavors of Linux.

If I turn on all the pc's, turn on all the wireless get all connected, and actually manage to surf the internet on everything, every machine can see every other machine, (except the phones), sometimes I can even access the shared files on most of the other machines. All but the XP, it is seen, and can see everything but nothing can access it, or it access anything. Which really isn't a big deal except that I want to add a large HDD, at least a TB or more, and then can use it as a "server" with enough space to backup everything, on one central storage space.

Up until two weeks ago, I also had a Belkin router, cascaded off the AT&T, because it has two usb ports, I had a 500gb external USB HDD plugged in for backups, and a non-network printer, so I was using it as a makeshift print-server. But, the battery back up the modem and router was plugged into shorted out, and fried both, AT&T replaced the battery, and the modem, it's up to me to replace the other router, I just haven't decided exactly what I'm going to do with that. But even before that happened I was still fighting with the same issues.

If I get things going right and I connect to pc-A, then connect to pc-B, I cannot re-connect to pc-A. Once that connection fails, I can't re-connect to pc-B, or pc-C or any other pc. But if I start out after rebooting everything, I can connect to basically anything, until I connect to something, and that knocks everything else off line. Occasionally, I can connect to each machine, before I get kicked off line.

So any idea what I'm doing wrong? I went through every pc, and turned off the homegroup, set-up every thing on the same workgroup, went through the C: Drive and individually shared each folder I wanted to be accessible, renamed all the folder so that it is easy to distinguish what pc it is from, for instance instead of having 4 folders all named "My Documents", I have 4 folders named "Docu-A, Docu-B etc..." When I open "This PC' on my 8.1 machine, or the "Network Neighborhood" on the 7 machines, I can immediately see all of the shared folders, sometimes I can connect to those folders, sometimes I get the "You do not have permission" error. But if I click it two or three more times, some times it will let me in. Why can I access the shared files some times, but not all the time?

I tried to think about everything you might need to know, but I feel like I forgot something, so just let me know what else you need, or what I made really confusing, because I have a habit of doing that. LOL.

Thanks for the help.