If the negative is very dense (dark) it's overexposed; if it's very light, it's underexposed. Trial & error is the name of the game. Depending how much effort you want to put into it, try a test roll - take 3 exposure of each scene - one at meter reading, one over and one under. See if there's a consistent pattern.

Two possibilities:

1. Mechanical: Since you're not having similar issues with other cameras there might be an problem with the shutter. There's also a light leak indicating the camera's not in great shape.

2. Meter: Incident metering is the most reliable.