Mixdown - distortions and crackles on certain tracks

Hi all. First-time poster here. I was wondering if someone could help me with a problem I haven’t been able to solve after five or six hours of experimenting with different solutions.

With a certain project I’m working on, when I mix the whole project down, a couple of tracks (two mono tracks of DI’d electric piano sent through a stereo buss) contain crackles and distortions. There may be crackles on other tracks too, but I’ve sent the vocal and guitar busses out as separate stems and they sound clean to me on playback.

The two piano tracks are peaking at around -15 dB in the tracks and about -13 in the buss. The output of the piano buss to the stereo buss is at 0, and the whole mix combined has an RMS of -26, with a peak of approximately -6.

The tracks sound clean when played back within Cubase but distorted when played back after mixdown, in whatever media player I use for playback (I’ve tried Windows media player, Switch, and Soundforge).

As I’ve been careful with gain scheduling, I don’t understand what is causing the problem. I thought that the problem might be with the cable used to DI the piano, so played the same part via midi, but when I bounced the project with the midi piano in place of the DI’d digital piano, the mix contained the same distortion. Whatever’s causing the problem, it’s got me beaten.

I’m using Windows 7 (64bit) on an HP Pavilion DV6 laptop. My interface is a Lexicon i-onix U82s, and I’ve tried reinstalling the driver for that (the updated driver for use with 64 bit Windows 7, which I obtained from Lexicon’s website), as well as doing a repair install of Cubase. Other projects I’ve tried do not seem to have the same problem.

Any help would be appreciated. Thank you.

Have you tried raising the sound cards buffer size/latency?

With the interface I’ve got, it gives you options ranging from ‘relaxed’ through to ‘highspeed’. I’ve always had it on the default ‘normal’ option, but today I’ve tried nudging it down, first to ‘relaxed normal’ and then ‘relaxed’ doing a mixdown each time. It doesn’t cure the problem.

Thanks for the suggestion though.