Generic dictionary emptying itself?

Anyone know why a custom dictionary, generated and populated in code, would work fine for a while and then spontaneously lose all its entries?

I'm making a card game, often need to move an object relating to a card suit, and rather than searching for the object by name (which worked fine) I thought a dictionary would be more elegant... and it works perfectly for a while, then seemingly after a few minutes of gameplay, starts throwing up KeyNotFoundException: The given key was not present in the dictionary.

Trimmed down code as follows: (JS/US)

 #pragma strict
 
 import System.Collections.Generic;
 
 var plates = new Dictionary.<String,Transform>();
 var pct:Transform;
 var pdt:Transform;
 var pht:Transform;
 var pst:Transform;
 //the transforms are allocated manually in the editor
 
 function Start () {
     plates.Add("club",pct);
     plates.Add("diam",pdt);
     plates.Add("hear",pht);
     plates.Add("spad",pst);
 }

Then later, I'll want to move the "pct" object using something like:

 function selectcard(){
 ...
 var suit="club";
 plates[suit].position=Vector3(0,1.6,-1);
 ...
 }

Apologies for not posting full code, it's over 1000 lines dealing with other objects and events.

As I said earlier, it all works fine for a while then starts throwing up errors. Outside of Start() the dictionary is only ever referenced to change the transform's position, nothing added to or deleted from it.

I'm certain the dictionary is being emptied as I've checked using

 print(plates["club"]); //and likewise for "diam", "hear", "spad"
 print(plates.Count);

...all of which return the expected results until the error appears.

Are there any methods not directly related to dictionaries, which might be doing this as a side effect? I'm at a loss, may have to go back to GameObject.Find every time I move these items :/