I want to imlpement a c# class (.NET 4.6.1) which contains time series data as follows :
When I first saw the question I immediately thought of a queue, but most built-in queues do not efficiently allow indexed access, as you've found.
Best suggestion I can come up with is to use a ConcurrentDictionary. Thread-safe, near-constant access time by key, you can key directly to DateTimes, etc. it's everything you need functionally, except the behavior to manage size/timeline. To do that, you use a little Linq to scan the Keys property for keys more than one hour older than the newest one being added, then TryRemove() each one (you can derive from ConcurrentDictionary and override TryAdd() to do this automatically when adding anything new).
Only other potential issue is it's not terribly memory-efficient; the HashSet-based implementation of .NET Dictionaries requires a two-dimensional array for storage based on the hash of the key, and that array will be sparsely populated even with 100k items in the collection.