Also good points.  How about this:  If a victim is able to endure the effects of the poison for a full minute then the effects cease.
		
		
	 
That's probably a nice compromise.  A rare occurrence*, but a solid compromise.
[sblock=* How Rare?]
Assume a Dwarf Fighter 5, with a 16 Con.  Fort Save is +7 / +9 vs. Poison.  Hit with Arsenic, always rolls a 9 on his save.  DC 13.
Round 1, make his save, lose one Con (15).  Save now +8 v Poison.
Round 2, make his save, lose one Con (14).
Round 3, make his save, lose one Con (13).  Save now +7 v Poison.
Round 4, make his save, lose one Con (12).
Round 5, make his save, lose one Con (11).  Save now +5 v Poison.
Round 6, make his save, lose one Con (10).
Round 7, make his save, lose one Con (9).  Save now +4 v Poison.
Round 8, make his save, lose one Con (8).
Round 9, make his save, lose one Con (7).  Save now +3 v Poison.
Round 10, fail his save, lose 1d4+1 Con (5 to 2).  Save now +2 or +1 v Poison.  Poison ends.
Assume a Human Fighter 5, with same base stats.  14 Con, +6 vs. Poison.  Hit with Arsenic, always rolls a 9 on his save.  DC 13.
Round 1, make his save, lose one Con (13).  Save now +5 v Poison.
Round 2, make his save, lose one Con (12).
Round 3, make his save, lose one Con (11).  Save now +4 v Poison.
Round 4, make his save, lose one Con (10).
Round 5, make his save, lose one Con (9).  Save now +3 v Poison.
Round 6, fail his save, lose 1d4+1 Con (7 to 4).  Save now +2 to +1 v Poison.
Round 7, fail his save, lose 1d4+1 Con (5 to -).  Save now +1 v Poison, or Dead.
Round 8, fail his save, lose 1d4+1 Con (3 to -).  Save now +0 v Poison, or Dead.
Round 9, fail his save, lose 1d4+1 Con (1 to -).  Save now -1 v Poison, or Dead.
Round 10, fail his save, lose 1d4+1 Con (-).  Dead
The Dwarf is a "best realistic case", the Human is a baseline hero case.  The Dwarf barely survived (hooray racial bonus against poisons), and will need 11 to 14 days to recover (or up to 7 Lesser Restoration spells).  The human died as early as round 7 (without rolling an early failure),  and was guaranteed to die by round 10.
Congratulations, by standard saving throw rules, you've got a very deadly set of poison rules.  I hope it works out well for you and yours.  Good luck.[/sblock]