Subject Fast Slow Stored procedure

What are the possible reason for a SP that normally runs in approx 20 seconds then Occasionally takes 15 minutes, (or a method of trying to find the cause of the problem).

Firebird Details

Server Version: WI-V2.5.5.26952 Firebird 2.5 32 bit Superserver, CpuAffinityMask = 1 in firebird.conf


Windows 10 Pro 64 bit

Processor Intel Xeon CPU E5530 @ 2,4 GHz Quad CPU

Memory 12.0 GB

Accessing DB using IBObjects components (TIBStoredProc)

Available tools IBExpert

Thanks for any help