Hello Kiwigirl,
This is a known bug, and sadly it hasn't been fixed yet. So yes, many other people have encountered it.
I think it is due to the fact that, at some levels, the work shift is spread overnight. I had similar issue with the medical career too, where one level is also overnight work.
I'm afraid there is no 'cure' as long as EA doesn't fix this bug in a patch. But, if you give your Sims the 'vacationer' LTR, it should help reduce the lost of performance when he is not at work.