Live in Costa Rica but commute weekly to US for work?

Unless you apply for Residency, you could be refused entry at Immigrations discretion...although this is unlikely. They are giving less time for some  tourist visas, which shouldn't bother you if you leave every week...but you just never know...

Being under 55 applying for  residency would mean a premium of approx $450 a month for mandatory healthcare costs which you may not use, so you could continue as you are traveling as a tourist.

You should take into consideration the increasing frequency of the airport being closed for hours and flights cancelled due to the activity of the Turrialba volcano. Many flights cancelled again, this morning.

That sounds like an interesting lifestyle.  Have you looked at the flights available from CR to your typical weekly destinations and factored in potentially increased travel times?

I have... flights are shorter and cheaper - I fly from west to east, so its a benefit to move to a closer time zone. Delta has two good flights out of SJO daily to Atlanta, where I can make connections.

Before you move know exactly what you are doing.    I believe if you leave the country every three months you can reside as a tourist.     But I agree with what others say there are road closures and cost of flying back and forth and up keep of home.   Do your home work or it could turn to a night mare