Playback problem with Rockboxed iRiver H140

[Yamahasi]

Hi
Looking for some advice with a problem with my iRiver H140. I fitted a 128gb ssd (Kingspec) drive a couple of years ago (did the same to my iRiver H340 as well. New batteries in both and both had been previously Rockboxed. Both worked perfectly and I was really happy with everything. However recently the H140 has developed a weird fault! Some tracks sort of speed up (bit like fastforwarding for a few seconds) at exactly the same spot. The same tracks play perfectly when the H140 is booted back into the original iRiver firmware.
I've swopped drives between the two players and the H140 still does the same but the H340 works perfectly, I've also reformatted the drive and it does the same thing in the H140 but not H340.
Currently I'm using the H140 with the iRiver firmware (EU 1.66 version) but I'm really frustrated by the lack of gapless playback, limited formats and all that wasted space on the drive - I've reached the 9999 song limit in about 56gbs, which is about half my collection.
I've Googled this for a couple of weeks but can't find any mention of it. Because of the swopped drives and the fact that the songs play fine in the original firmware I'm assuming that it is a problem with Rockbox or the H140 (rather than the drives themselves).
Has anybody any ideas or is it time for the H140 to be retired?
Thanks in advance for any help or advice.