Problems making audio cd's (nero)

[FWTBT]

I use NERO to create music CD's (converting MP3's to CDA format), and recently I've noticed a few CDR's fail to be read in my HiFi (the HiFi is an AIWA HiFi). Most CDRs I make work, yet some do not, and I've only really noticed this inconsistent performance recently. I have always used Tevion 700 MB CDRs (40x max.). I burn them with my LiteON 40x12x48 CD-RW.

Could this be my ageing HiFi failing to reconise the CDRs? Or could it be the burn format (Disc-at-once), or the fact that I'm burning at 32x? I have burnt other CDRs at 40x (Tevion cd's), and they work on my HiFi, yet for some strange reason some (my last 4 attempts) do not. Should I consider slowing the burn speed down to under 24x and/or use the "Track-at-once" option to burn? Does the problem lie with the particular CDRs I'm using (bearing in mind I have not had these problems all the time, although Tevion seem to be rather inconsistant as of late (in terms of burns that work on my HiFi)), or could the problem be with the AIWA hifi (it's a 3 year old model).

Help!