I think you are looking in the wrong place. GFCI doesn't pop under load--that's the breaker's job. GFCI detects an imbalance between the poles and shuts down. I think the GFCI is doing EXACTLY the job it's supposed to do--shut down when there's a ground fault.

It sounds like you have a short somewhere in either the lights or the Jandy dimmers. I would start testing there. It also sounds like it's an intermittent short, too. I would STRONGLY recommend you not use the pool until the problem is solved or the lights are completely disconnected from all electricity, either by unplugging or if hardwired, removing the wiring.

Why do people always assume that when a breaker or GFCI keeps popping its a bad breaker or GFCI, not that they are doing their job of protecting you?