Is it permissible for me to occasionally spend from my deceased father's money, of which I am the agent for his pension, on myself?

If the guardian does not have the right to dispose of the salary, and the guardianship does not authorize him to benefit from it, then it is not permissible for him to spend from it on himself. Moreover, consuming an orphan's property is among the major sins. Conversely, sponsoring an orphan carries immense reward.