1 wire alternator ?'s

[67 C-10 LWB]

I bought a 100 amp Proform 1 wire alternator from jegs when I rebuilt my engine about 2 yrs ago.
too hook it up I just took the wire to the battery and hooked it up to the alternator output post, I left all the other wires unhooked, and w/ with the external regulator I just unplugged it, didn't do the jumping of terminals like I've seen here.
The question I have is this, when I first fired it up it was fine, it kick to full charge around 1200-1500 rpm like they are supposed to do. lately, and this is since like last fall i would say it won't kick on till about 3K rpm when it's cold, but once it "excites" that one time it will come on around 1200 or so, any ideas? is it because I didn't bypass the old external regulator, or is the alternator going bad? any help would be great, thanks

Ean