Hello all! I'm new. Maybe someone can help me?

I have a 1/10th scale awd Helion Dominus with the Castle Sidewinder 3 system and a different pinion gear. I forget the size but the truck now tops out at 35 mph rather than 24 mph.

Problem: For about the last week, every time I charge up the battery (5,000 NiCad) the truck runs at about half power for the first 5 minutes of driving. Then it will pick up and run much faster after a few minutes. Any idea what is causing this?

Thanks!