Murphy & Summer's Dad is not only the founder of this site but very knowledgeable about these things. I know you said you don't have access to a vet but is there any way at all???
In the meantime here's another site you could look at
http://www.merckvetmanual.com. You will never know for sure until you can get a vets opinion but on this site there is info on spinal cysts and other spinal problems.
If you absolutely cannot find a vet give us very, very detailed descriptions of what it looks like, when it happens - details - does he show pain in his spine when you apply pressure, does he lose focus in his eyes, what exactly is he doing when it happens - look for a pattern or type of activity like jumping, running, rolling over, etc. Do his legs bow or turn out when just standing still. Give everything you can think of. I will ask my vet for what the most likely causes might be-keep in mind he cannot diagnose without actually seeing the pup so you will only be given directional info. (I am going to go to work for this vet later in the summer/fall so they will be happy to help me help you)
Can you video tape his play and send an mp3 or clip of it?
Hope to help...