HVAC Blower randomly goes to lower speed

I've been noticing my hvac blower randomly go to lower speed all summer. I'll set the speed of the blower manually instead of relying on the auto climate. Then usually about 5-10 mins into my drive the blower will go to lower output but remain at the same level on the dash. Randomly...5-10 minutes later it will kick back into it's original, higher speed. Reading the forums it sounds like the blower motor fails frequently due to water. I replaced the blower motor and still experienced the problem. Then, I thought it would be the blower mower resister/final stage unit. I replaced that as well yet still experience the same symptoms. Any ideas what the root cause could be?