If i take a job requiring me to work outside the Netherlands 4 or 5 days a week but get paid through a Dutch staffing and payroll agency (and keep my apartment here) will I still be eligible for the 30% ruling?
If you remain on the Dutch payroll administration and employed by the employer with whom the 30% ruling was granted you can indeed still benefit from the 30% ruling while working abroad. This is however not the idea behind the 30% ruling and the reason why it was created, but it will not prevent the use of the 30% ruling.
On the other hand, the country in which you work can argue that you should not be on a Dutch payroll anymore but be payrolled in the country where you work. Tax treaties state that income from employment in principle is taxed in the country where the work is performed. This can be different if the so called 183 days rulings can be used whereby you are working in the other country for less than 183 in a (calendar)year and are not paid by an employer or entity in that country.
So in each individual situation has to be determined where the payroll administration must be set up and whether the 30% ruling can still be used.