Speed spikes when I stop pedalling

You don't say what sensor you are using. You may want to check the alignment of any magnets, and of the sensor itself. Deleting then pairing the sensor again also resolves many problems. Replacing the battery is also a suggested solution to sensor problems.

Of course, the simplest solution would be just to remove the speed sensor - the Edge will revert to GPS speed.

I guess the latest ones, I bought a 530 sensor bundle, its my first bike computer and I can't say I'm impressed.

First issue was the auto start feature which I had to turn off because the gps drift was so bad it registered as an activity (I saw 25kph at one point).
That and the speed spikes really makes me question my purchase. I was using a forerunner 220 before this and frankly it worked way better.
So your suggestion of removing the speed sensor had already crossed my mind, but I'm a bit stubborn cause I bought an upgrade, at a premium price, something specifically designed for riding and it doesn't seem to work as intended.
I do like the gps fucntion which was the biggest motivation of buying one, but it just bugs me you know...

But enough ranting thanks a lot for trying to help me out, I've got a few idea's I wanna try out including a battery replace.
I'll update with my findings when I get round to them, he types sporting his olive "procrastinators unite" T-shirt his sister bought him because he keeps putting off buying new clothes... ;-)