The car has almost 90,000 miles on it and depending on how the clutch was treated, the adjustable mechanism may be out of space to adjust itself. It may be getting time for a new clutch. Also, the cable could be stretched causing the adjustment space to be used up prematurely. Is the clutch pedal really hard ? If so it may be time for a new clutch.
I would consider replacing the clutch cable and stock quadrent with an aftermarket adjuster and quadrent as well as a new OEM cable as a first step. I like the UPR adjuster the best so far of all the ones I've see. Don't get the Maximum Motorports adjuster, I have it and I'm going to be replacing it with the UPR unit. The MM, UPR and Steeda quadrents are all decent. I will say that I have seen the UPR quadrents brake where the cable attaches.
I know on my Mach 1 I reached the end of the adjustment and my dad's Bullitt did as well.
I have seen the shuddering in first gear you described happen when the clutch doesn't smoothly engage. Usually clutching out and back in will solve it. If you clutch adjustment is bad and cauasing you rough engagements on a consistent basis, I'd again say either replace the stock unit with a firewall adjuster or maybe it is time for a new clutch.