Welcome to the TD Replica forum Dick.
Here’s my experience with these replicas and I’ve had a VW based car, Ford based and now have a Chevette based BCW kit like yours.
Your problem starting at 65 mph (quite brisk for these little cars) is probably a combo of two things, too stiff of suspension coupled with aero lift. That’s right, lift.
At speeds above 50 mph the front fenders catch a lot of air. The front end gets “lighter”. Coupled with OEM Chevette springs that are a tad too stiff, the steering in you car will get “twitchy” as the front of the car gets “lift”.
I’ll bet it’s fine at slower speeds. The Pontiac Fiero uses the same front end components as the Chevette/Pontiac T-1000 and due to the mid engine configuration, their front springs are a little shorter and softer. They are the same ID and OD as the Chevette spring and will fit. It’s trial and error if you don’t have a set of scales and a wind tunnel to dial it in.
Keep in mind that these cars are not 21st Century design and will never ride and drive like a Miata or BMW Z3 or Z4. Believe it or not, they are pretty much a post WWII rudimentary sports car. The sport was having enough nerve to go fast in one!!!