1. How much was the block and/or heads decked? Even a small amount can change the timing angle in these cars, and it is not equal between the driver and passenger sides due to the V-geometry. 2. Did you install any adjustable timing gears? This can help compensate for the decking, but can also be the cause if not set correctly. 3. Of course, I'm sure you double-checked to make sure your timing belt is not off a tooth, but that's the most obvious reason this could happen.