i've always had better and more consistent results with sculpey than fimo.
i bake my pieces at 250 for about 40 minutes - cooler and longer than the
directions say. for thicker pieces i use the same temp and even longer
time - no burning/browning of lighter colors and they always seem "done".
sculpey makes a flexible-even-after-curing variety for things that aren't
best brittle - bracelets, etc. maybe that's what you got? i have had some
difficulty with sculpey that is really old...