Disconnect exterior fuel tank hose. If fuel flows fine by gravity. One could assume the fuel tank filter is not obstructed.

Years ago we had a vehicle in that somehow their seemed to be some bunker C in the a gas tank. Periodically it would cover the tank filter it seemed.

Also make sure it is not a vacuum developing in the tank by leaving the fuel cover loose. Both are pretty simple to eliminate as potential causes. Not that they are the cause.

Both are actually unlikely yet the tests again are easy. Pulling out a tank filter you can prove is probably quite serviceable otherwise is a messy issue. Plus perhaps more work than is needed.

Personally i would run a hose back inside the car to monitor the fuel pressure in the base of the injection pump. Is it staying up or far too low. Another quick attempt to eliminate the injection pump perhaps. Is to close off the return line from the injection pump. If there is no change at all with that line closed off. You do have some issue in the fuel supply system. If it is enough to cause the surging is another thing.

Generally speaking injection pump failures of this type must be considered pretty rare on these old cars. Not impossible yet rare in my opinion.