Is the surface of the floor smooth and, although not level, is it even?
If not sort of suggests it is crap.
If you knock on the floor, does it sound hollow?
If so, means there could be voids below. Again, suggests not laid very well.
Does the room have obvious places where skirting boards have been replaced/reattached due to movement of floor relative to walls?
If so, suggests a possibly serious problem.
To be honest, digging it out and replacing, while a lot of work, mess, money - might be the best choice by far. Surely anyone who was competent would never have laid a floor as out of level as you report? But there is always a chance that it was done like this for a reason (albeit not a good one) and you will find out why as you replace it.
You do need to get advice on exactly how to construct the new floor. Others here know far more than I do about the best techniues.
There are lots of photo sites such as Flickr. I can't remember the last one recommended here. Search and you will find one. Upload and post links.