DBA Required for long query execution time (C# winforms application)

We have a C# winforms application working with a SQL Server 2016 database. We have some specific queries that execute quickly but will suddenly take a long time to return the same results. The tables don't even have an excessive number of rows. We can't understand why these queries are sometimes very fast but for the majority of the time very slow. The server is not struggling, it has plenty of power and memory and CPU and memory usage is low.