Kezza
I think you've got it back to front. If you open the aperture, eg more light, it will give you a faster shutter speed not a slower one.
Even with the FZ8's OIS shake reduction system, unless you have an extra steady hand, camera shake will occur at anything slower than 1/30 or 1/15 of a second. As you increase the zoom, then a faster shuuter speed will be required to avoid shake.
Regarding OIS, make sure it's switched in, it does help.