Go frame-by-frame through a movie with a precise timer

If the sync error remains constant then you have a delay issue.

If you are using version 0.8.2 or above then keys g and h will allow you to adjust the sync delay during playback.

If your subtitles are appearing and disappearing too early then use h to increase the delay until they appear at the correct time.

If your subtitles are appearing and disappearing too late then use g to decrease the delay until they appear at the correct time.

If the sync gets worse (gets more and more ahead or behind) as the playback progresses then you have a timing issue.

This is usually due to a subtitle file being set up for a different frame rate than the video.

You will need to convert the subtitle file to match the frame rate of the video.

As you are using .SRT subtitle files there are some suggestions on tools to re-time them on this SuperUser question: Which is the best subtitle file editor (srt)?

You can use MediaInfo to check the frame rate of both video and subtitle files to help you decide which conversions are required on the subtitle files.

If your video happens to be in mp4 format then for example MP4Box can be used to demux (extract) the different types of streams (video, audio, subtitles) from it. This works only if subtitles are not actually "burned" on the video image (easy test - can you turn them off in your player? If yes then all should be well).