Hello: my rain gutters seem to be overfilling more frequently than normal during heavy rains.

I recently moved into a 1965 house with the normal set up of down spouts on the 4 corners. The house footprint is 55 x 35 and I assume the house is designed with rain gutters & 4 downspouts that can collect and drain water in a “normal” rain, but the “system” would be overpowered by a “heavy” rain. In Northern Virginia, average rain is 3-4” EVERY month and with high humidity, thunder showers are more common than just “steady” rain. However, it doesn’t seem right to see water pouring over in heavy rains and we’re concerned about unseen damage/rot from this.

I do have some issues: the house is on a down slope and thus, the basement is walk-out on the back. The 4 down spouts tie to an underground drain system of some sort. We checked the gutters and they look clean; a handyman also ran water down the spout and it appears to be draining (although I don’t know the rate or amount of water being used). Where it is overfilling, we took the downspout off the underground drain because we suspect the drain pipe was made from clay/ceramic and it may have collapsed in some areas or gotten clogged. Still, in heavy rain, the gutters overfill and water pour over.
The previous owner said he replaced the gutters 7 years ago and I assume they were done properly.

Heavy rain also doesn’t always happen at daytime when I can get a better look at what’s happening. What are the steps to take to diagnose the problem?