Re: new GL3 1086-4....casting problems
If the collapse is on the forward cast, I suggest you follow the same step-by-step process I outlined for the backcast except do it for the forward cast.
Set the line out behind you just as if you had stopped the backcast. Once you've setup your stance, rod to the rear at about 2 o'clock, casting arm extended, grip, and cocked wrist, begin the forward cast -- do this by retracting your arm gradually accelerating the speed. REMEMBER: the hand leads the rod forward until it's about even with the body; the palm should be facing upward so that you cannot see your thumbnail.
As the rod passes to your front it's time for maximum acceleration (FlickStop). FlickStop should stop the rod momentarily at about 10 o'clock. Once the tip has been stopped, continue to follow-through until you've reached what ordinarily would be the "start" position -- rod tip down ready to fish.
I suspect the reason the cast is collapsing is that you are failing to follow-through at the end of the cast. This typically results in the line falling to the ground with lots of slack. The tell-tale is the rod tip up. Is it up or down?
You should practice separating the fly cast into its two parts until both are performed faultlessly.