My passport gives me 90 days visa free travel to S'pore.

I'm planning to use S'pore as a base for doing business in SE-Asia for 6 months - meaning that I'd be out of the country once or twice a month during this 6 months' period.

Would immigration be OK with this (starting the 90 days visa free at each entry) or would it become an issue?

I would not do any "on-shore" business in S'pore, just with customers in other countries. My salary will be coming from my home country.