A random number generator actually returns the next number from a long sequence of 'random' numbers that takes a long time until it repeats the sequence. The 'seed' just gives the starting point in this long sequence. So you should only call it 'once' (say at the start of your 'main') - rather than every time you ask for the next number.
That's how the "usual" random number generators work. Good random number generators used in security applications use unpredictable external entropy generators. I can give you two examples for this: The Linux kernel includes two random numbers generators, you can see them throw the /dev/random and /dev/urandom pseudo-devices. The /dev/random generator uses entropy to give very good random numbers (this entropy comes from things like ethernet packets and other i/o activity). It can't give an infinite number of random values (if you request random numbers fast enough it will eventually run out of "entropy" and block until more entropy is acquired). The second example is from TrueCrypt OR the putty ssh suite, don't remamber exactly. When it needs to generate a random private key it asks you to move the mouse over the window to "acquire some entropy". That's very smart because you can't possibly move the mouse in the same pattern twice, so it gives good entropy.@ross75
Using a photoresistor as a entropy generator might not be a good idea since the photoresistor might generate the same value over and over again (if the robot doesn't move from it's point, if there's too much light and the photoresistor is saturated, if there's too dark, if the light levels in the room are uniform). Unless you require unusually "random" random numbers it would also be much slower since analog to digital conversion doesn't happen in an instant.
In my opinion you should not bother "fixing" the random number generator unless it is annoyingly broke: Example: if your "robot" is dealing poker cards and allways gives 4 aces to your opponents, if your robot uses the random number generator to select a different direction to travel when it encounters an wall and allways selects the same direction.