The problem is that mpega decoding saps the CPU. Drive access and image decoding will need some CPU power, and so mpeg playback is not that consistent.
The solutions I can think of are:
-Use a PPC mpega.library (which may not help much due to context switching)
-Drop the quality of the mpeg decoding. AmigaAMP and AMPLifier both have settings for this.
-Use a hardware mpeg decoder like Delfina, Prelude w/ MPEGit module, or MAS-Pro. There's probably others too, but those are the most common.
The third option will produce the best results, but is the hardest to implement due to availability and pricing of said hardware.
