Well, my Mkiv navicomp was changed under warranty for the same issue - believed to be a dead cooling fan. If you have warranty then you have absolutely nothing to loose in taking it in and complaining that its faulty.

TBH, I had no issue - they just ran a few diagnostics (apparently) and then agreed to swap it out - though they did order a Mk3 in by mistake 1st go.

See if you can talk them into ordering in a Mk4 DVD unit, but I very much doubt they will entertain the idea. Of course you could just get them to fix it under warranty and then in August buy a Mk4 off ebay (get the RIGHT Mk4 tho) and sell your Mk3 on.