Thanks for the shift registers idea! The errors I get do seem really similar to yours, G_Man though I'm not sure it's the same issue since I'm only using the LiquidCrystal library and the tlc5940 library (not the Wire library). Also, the way my circuit is made, it's really hard to add in new components and do any major circuit changes. However, I will try this idea out if it comes to it!
For my LCD display I'm using the circuit/library/code here:
And for the tlc5940 I'm using the circuit/library/code shown here:
http://www.arduino.cc/playground/Learning/TLC5940
Except with three tlc's linked together.
So basically I'm just running an LED display with the tlc's (which, again works wonderfully all the time). And then I just try to run the sample "Hello World" lcd code and everything starts behaving unpredictably. Mostly I just see a row of boxes although several days ago I managed to get some garbage like "32[][]" and then the first word I coded...
I'm wondering, has anyone else has had any issues dealing specifically with combining the lcd display and the tlc5940 driver?