Final Cut Pro : Movies stops

I am working with FCP 6.0.6 (iMac-OSX.5.8) since more than 8 years without problems. In the last months I have made some movies (format H264 and fullHD) and I find some brief stops during the screening. Is it possible that the software is damaged ?

I put the cache preferences in the trash and repaired the authorizations but the problem is always there.

Please help me because I must finish a commissioned work.

Elsa