Two things:
1. If you set NLS_LANG to us7ascii on export (we8iso8859p1 charset on db) that would not do you any good - will get another error. Just do not set NLS_LANG at all on export and run export with log option - you will see that export will be done in us7ascii by default! On import do not set NLS_LANG at all either.
2. There is a bug between 8.0.5 and 8.1.x on export/import (don't remember though whether it's from 8.1.x or from 8.0.x...). But workaround is as follows: do export/import through network. The bug is because executibles are so different that there is no fix - just a workaround. I would have found this bug for you - but you are right - Metalink is so slow today!!!
Alla