TTN3 B/T GPS crashing

Hi, have been using the TTN3 on a HP rx3715 using the TT b/t GPS since x-mas (I have got the new TTN5 and will shortly upgrade). In the UK it has been OK but has lost fix a few times which I reported to TOMTOM who suggested to upgrade to the latest 3.07 software which I have not done as the PDA says its already installed.

Anyway to the point, I went to France earlier this month and had major problems with the GPS loosing fix every 10-15min's on the way down south, I put it down to over heating in the car window as it was V.HOT and I have no a/c in the car. I made up a small 'paper hut' as recommended elswehere and it helped a bit but in the end I had to move the GPS out of the way into the car to try and keep it cool, this helped a bit but still not reliable to navigate by as it still kept loosing fix every 10-20 minutes.

I switched maps in France (major roads only) and it worked OK for a while until same thing happened, got feed up and used a paper map and then let
the TTN3 run on 'no guidance'. Every so often I got it going by turning the PGS off-on for it to loose fix after another 10-15minutes. Put it down to hot weather again (it was 28-30 going south). On the return journey it was cooler 22-23 so expected the system to work well in a 'no giudance' mode but no it was the same all the way to the ferry.

Back in Portsmouth, I returned back to UK maps and it navigated perfectly all the way into Scotland !!!!! (weather was cooler at 16-18 and no sun)

Whats going on ?
1/ surely a GPS designed for a car should cope with the heat, especially as I ran it will a charger to keep power at full capacity.

2/ Why did the system run so poorely in France even when the weather was cooler.

I am going for TTN5 and hope that things will improve but to me the B/T GPS is the weakest link if it can not cope with some heat. What will happen next time I go into Europe or it gets really hot, do I have to get paper maps out again?
Any comments would be very welcome as the unit is still under warranty.