I have it set Mail to randomly select a signature
( can no longer even find this feature),
It's one of the options in the Choose Signature popup menu that appears at the bottom of the Preferences > Signatures window. To make this popup menu active you must have an account selected in the list on the left. It seems that you cannot change this setting globally for all accounts AFAIK, you must change it for one account at a time.
but it keeps selecting a few
and rotating through them, as if on a loop.
Maybe the option chosen in the Choose Signature popup for that account is "In Sequential Order" instead of "At Random".
when typing a new e-mail, I have attempted to add the
signature of my choice manually, but my list will
only extend to 42 signatures only. I have 59.
Since you say Mail does show all signatures when the account is selected in Preferences > Signatures, I suspect you're encountering some kind of corruption in Mail's signatures database. I would try this:
1. Quit Mail.
2. Delete the Signatures folder in Home/Library/Mail/ (or just move it to the desktop). If you have a Signatures.plist file in Home/Library/Mail/ I would delete that as well.
3. Recreate the signatures in Mail from scratch.
BTW, you're not alone, see
Mail Signatures Suck.
iMac G5 17" (iSight) Mac OS X (10.4.4) 
