How to Identify a Best Ecommerce Websites Development Company?

I sell handmade perfumes and last year I paid a company to build my online store. The site looked fine at first but after three months it started crashing every time I added new products. Customer orders got mixed up and I lost many repeat buyers. Now I am searching for a best ecommerce websites development company that builds stores which actually last. What specific things should I check in their past work? Do I ask for a list of clients I can call directly? How do I know if their code is clean or just a quick fix that will break later?