If there is a change in the way the system operates when it getw warm then I’m afraid it is likely to be a problem with the hydraulic system itself.

Basically when the oil gets warm it becomes less viscous (it becomes thinner) and hence more likely to leak past any components.

It is possible for the problem to be something simple such as an o’ring on the stand pipe, but a leaking o’ring may well also leak to quite an extent when the oil is cold.

It is more likely to be wear in either the hydraulic pump and/or control valve. It could be the relief valve or a problem with the cylinder, but usually the pump and control valve. A pump repair kit isn’t all that expensive and the DVD shows how to refurbish the pump correctly.