permalink confusion

Hi
Im having problem with my wordpress web site..
I will start by saying I know very little about permalinks..
On my site I always used the “post name” permalink setting but now for some reason the links have suddenly started crashing…only home page loads up..
however if I set the permalinks settings to “default”.. it works fine
again
well why dont I use “default” I hear you ask!
well Ive already made pages for clients to view work that will not work if I use “default”.
my site is a purchaced theme and came with “post name” set
one last thing when my site pages crash …I set back to permalink setting “default” and then change back to “post name” it works fine
for a day or so then crashes again..

hope you not as confused as I am!!