help diagnose this fuel-filler overflow issue - 202

[Will_w202]

97 c230.....pump has been cutting off at 6-8 gallons for 2 years, worse in hot weather. I know when my car is full, and what point on the fuel gauge correlates to how many gallons it needs.

Except today, when it started dripping gas somewhere behind the wheel. Apparently overfilled. I opened the flap and there is no leakage there. This has to be related to the early cut-off issue I think. Where's the failure point? Is there a clog in the neck? I'm not sure what's between the cap and tank.