Without seeing code, it is pretty hard to say conclusively what is happening. Although, most likely the IDENTITY value is being cached, causing gaps in the value after SQL Server is restarted. See http://stackoverflow.com/questions/17587094/identity-column-value-suddenly-jumps-to-1001-in-sql-server for some good answers and info about that.
A simple INT field can hold values up to 2,147,483,647. You can actually start the identity value at -2,147,483,648, giving a full 32 bits of values. 4 Billion distinct values. I doubt very much you're going to run out of values to use.
Having said all that, if you wanted to use BIGINT as the identity field data type, there is certainly nothing wrong with that. That'll give you for all intents-and-purposes, a limitless supply of values to use. The performance difference between an INT and a BIGINT is practically non-existent, and highly preferable over for-instance using NEWID() to generate GUIDs.
If you wanted to manage your own values for the ID column, you could create a key table, and provide a pretty bulletproof way of doing that using one of the methods shown in the answers on this question: Handling concurrent access to a key table without deadlocks in SQL Server
The other option, assuming you're using SQL Server 2012+, would be to use a SEQUENCE object to get ID values for the column. However, you'd need to configure the sequence to not cache values. For example:
CREATE SEQUENCE dbo.MySequence AS INT START WITH -2147483648 INCREMENT BY 1 NO CACHE;