My reward for the same block is consistent with other blocks.  I don't think that it is a pool issue.  You probably need to check things at your side, for example - how is your mining equipment connected to the internet?  If you use WiFi - is it possible that your connection suffers temporary interference or load due to other users?  As mentioned before, Slush uses a unique 'anti-pool hopping' algorithm, which quickly reduces your score (and reward) if you stop/reduce your hashing rate towards the end of a block.  If you cannot solve this problem you may want to explore using some other pool which is more tolerant of such hashing fluctuations.

Cheers