If you place a scale, any scale, on an uneven or soft surface, you will not get a reliable reading. You could spend hundreds of dollars on the snazziest, most feature-packed scale in the world, and if all four feet aren't resting evenly on a totally flat and hard surface, you will have exactly the same problem as you are having now.
My (relatively cheap) Salter digital scale is very reliable, in the sense that when I place it on a totally flat area of my bathroom floor, I can get on and off it and the reading is exactly the same. If I move that same scale around to an area of the floor where it is just slightly uneven (where you can't really perceive it with the eye, but you can feel the surface isn't totally flat), the readings given vary drastically (can be as much as 5lbs above or below the reliably measured weight). This isn't a fault with the scale at all, it's a fault with the way I'm using it.
Obviously some scales are more sensitive than others, and there are things that scale manufacturers can do to mitigate somewhat against this problem (e.g., providing feet that you can adjust if you have to weigh on an uneven surface like a carpet). Unless you've done this already, I would see whether your scale is still unreliable when you are placing it on a completely flat surface (put it on a totally flat area, weigh yourself several times in a row standing with your feet in the same position each time, and see whether the number is the same or different each time). Weigh yourself on a hard surface, and definitely not something like carpet - manufacturers often claim modern scales can weigh on carpet, but it's still not nearly as accurate. If it's still giving wildly different areas when you're weighing yourself on a bit of floor that is totally flat and hard, then definitely see if you can replace it!
I'm really sorry if you know all of this already, but I had this exact same annoyance myself for a while, before I realised there was nothing wrong with the scale at all, but I was just using it incorrectly by putting it on an uneven surface.
major goals: healthy bmi @ 159 | original goal @ 150 | ultimate goal @ 138
stone goals: under 12 stone @ 167 | under 11 stone @ 153 | under 10 stone @ 139
c25k: completed! (started 30.08.10, finished 08.11.10)
30 day shred: level 1 day 1 2 3 4 5 6 7 8 9 10 | level 2 day 1 2 3 4 5 6 7 8 9 10 | level 3 day 1 2 3 4 5 6 7 8 9 10