P0325 Fault on 1997 Toyota Corolla: Knock Sensor Issue and Engine Knock Diagnosis

Hi everyone! I have a 1997 Toyota Corolla DX with 137,000 miles. Last week, the "Check Engine" light came on unexpectedly. I replaced the knock sensor, and immediately after the light turned off. However, about 15 minutes later (after driving approximately 10 miles), the check engine light returned, and a P0325 code appeared during scanning. After disconnecting the battery and restarting, the same pattern repeats: the light goes out for the first 10-15 minutes, then comes back within half an hour, triggering the P0325 fault code again. I know that the knock sensor detects engine knocking or pinging. However, I'm unsure what is actually causing this issue. The car is now only achieving around 20 MPG on highways — significantly below its expected fuel efficiency of 30 MPG. When driving uphill at a 20-degree incline at 70 MPH, the engine lacks power and the speed drops sharply, which feels worse than typical underperformance for this model. I’ve already tried cleaning the MAF sensor (internal wiring), and while the check engine light went out temporarily, it returned after about 30 minutes. I also reset the system by disconnecting the battery — same result: no light for first 15 minutes, then return of the P0325 code. I can now hear knocking noises when I place my head near the engine. Could this be a fuel mixture issue? Is it possible that the MAF sensor is causing or masking the problem? What should I do next to diagnose and fix this properly? Any advice would be greatly appreciated.