GTMetrix Website Health, what is causing my website to load slowly sometimes but works fine other times?

my website appears to be loading normally (within 3 seconds) sometimes, then other times it takes about 25-30 seconds to load. I cannot understand why this happens. I've attached some screenshots, and the link to the GTMETRIX result is: https://gtmetrix.com/reports/www.docbot.org/m3KZ2qXV

Could someone help me out please?