Hmmm The only time I think Lactic Acid has really effected me is running a marathon, 20 miles it felt like my legs were on fire.
In MX riders can pump up after a couple mins when there not even out of breath.
I dont think the body will switch up to a *secondary lactate energy source that easily. It's way more likely muscles tension, from not being comfortable enough on the bike causes a blood flow traffic jam. The blood just want to deliver oxygen & glucose, but the muscle is tense, once the back up begins to occur it just exacerbates from there, we then ride more tense etc.
Improved fitness will help, as you will be able to move around on bike better. But ultimately you need to improve your talent.