My 2009 Aveo5 LS manual transmission recently started having intermittent acceleration issues in rain. This issue doesn't always occur when it's raining but I notice that when it does occur that it's in heavy rain.
When driving at highway speeds in 5th gear in heavy rain the rpms will drop for about 4-5 seconds even though the accelerator is pressed to the floor (as if the TPS was unplugged), and after the 4-5 seconds power is restored again. During those 4-5 seconds, pressing the accelerator doesn't stop the rpms from dropping, but if I press in the clutch I can rev up the rpms . This "on and off" cycle of having no power for 4-5 seconds and then having power again for 4-5 seconds usually happens for 4 to 5 cycles before everything operates like normal again.
I've only noticed this cycle of power/no power occur once per trip, so I may continue driving for another 15 or 30 minutes and it doesn't occur again during the rest of that trip. This doesn't occur in light rains or in any gear lower than 5th, but only at highway speeds in 5th gear in heavy wet rainy weather. No CELs have ever triggered at all, so when I took the car to a mechanic to have it checked out they didn't know where to start to troubleshoot the issue and weren't able to recreate the issue since it doesn't always occur.
Has anyone had anything similar happen that they were able to successfully resolve, or does anyone have any idea of what may be causing this?