Well done for this again. I need to try have afiddle around with this again as I don't think anyone else has reported problems - so hopefully it was something else I was doing. But basically I installed this and on enabling the transmitter, the N900 completely froze. After about 30 seconds and still no response, I decided to try turn it off. When holding the power button down it did restart itself. Going near the transmitter caused things to slow down - but it did work.

After a full reboot (me switching it off then on) it seemed a lot better, but I still believe it was slower - maybe I'm just paranoid though!
I managed to call Nokia in the end and apparently my UK device is actually for the Netherlands and I therefore need to send it in to get replaced once more! Though I am wondering whether to just use this fix instead and keep it.

I uninstalled it in the end as I needed to speak with Nokia about the disabled FM transmitter fault (being a [supposedly] UK device).
When I get a chance, I'll see if the problem crops up again on reinstalling - or if it was a one off!

Just so you know - I'm far from dissing this - I just thought I'd give the feedback from my situation. This has been a lot better help than Nokia!