Is it just the one brand of blank DVD, or different brands?
Obviously we can rule out the drives themselves (as far as any malfunction goes, anyway), since you’ve changed drives.
Any other burning software on the PC besides Nero?
You don’t mention actually burning to the blank discs, what happens there - any error messages or anything?
Just to rule one other thing out: what speeds are the LG and LiteOn burners, and what speeds of discs have you tried?
Trying to cover most, if not all, bases here.