Error codes - Cat & Cam sensor

[Morph]

About a week ago my check engine light came on ( 97 318i, 86K miles) - local dealer said it was a bad Cat. Cat replaced with a free flow aftermarket unit and light reset by independent shop.
Just yesterday the check engine light came back on and a friend of mine connected my car to his OBDII code reader and engine scanner.

Here are the results :

1.) Camshaft postion sensor circuit malfunction.

2.) Catalyst Efficiency below threshold.

Reset engine light and everything is fine for now - no more code errors and car seems to run fine. Before I replace the Cam sensor, and look further into the Cat. and O2 sensors, is there anything else should be looking for at this point. BTW the Cat manufactutrer says that they do not have problems with 4 cyl. BMWs as far as check engine lights coming back on and suggested I look into the O2 sensors. If these were bad wouldn't I get an error code for them also ? Guess I'm wondering if there is a strange electrical gremlin or if the parts in question are bad ?? Any suggestions ?