Balicana in Los Corales. Not just Thai but other East Asian including Malaysian and Indonesian. Closed Sundays, no credit cards. Weekday lunch menu is great value at RD$300 pp. A la carte menu is also quite reasonable, the food is excellent, nice setting, wonderful hosts.
There is an Asian restaurant called Serai in one of the plazas between Cortecito and Friusa which also serves Indian food but I haven't tried it yet.