Hello,
as for memory, Parallels allocates Virtual Machine's RAM as one integral file. Consequently when your Mac’s RAM is too defragmentated (when too many applications are launched) then there could be not enough space in one sector for this file. I mean Parallels takes RAM (previously allocated to the VM in the settings) as one piece, not from here and there getting the neccessary amount all in all. So, as you can see, if you allocate not too much memory to your VM it would be easier to find a whole piece of this amount in the whole RAM when other applications are running (btw they take RAM by pieces not one after another but in different parts - that's why I called Mac's RAM defragmentated).
jvalal, sre you trying to set RAM for the VM or for the whole PD?
Best regards,
Alicia.