This could be a problem with the filament rather than the printer.
If the filament contains air bubbles, they will heat and pressurize. The additional pressure can increase the flow rate for a moment as the molten material in front of the bubble is expelled. The flow rate decreases after the bubble pops through the nozzle because the now-empty space needs to be filled by the extruder pressing new filament into the melt zone.
If this is happening, you might hear little pops.
Alternatively, the filament may have absorbed water, which can convert to steam and explode out the nozzle like a bubble would.
Drying filament isn't hard. I use my household oven at it's lowest temperature, which is typically about 160 degrees Farenheit. This is high enough to slightly soften PLA, so don't go any high than you must. Bake the filament for an hour, let it cool before handling it and try again.
The filament may not be quite as round after baking, but the cross-sectional area shouldn't change.
If the problem is from bubbles, there isn't anything you can do to fix it. If it is from water, you can fix the filament. I'd suggest baking the filament and seeing if anything changes.