Thanks for you reply.
Thats a reasonable response to a fixed frequency error. OSCCAL can be set to change the internal oscillator frequency.
VirtualWire works by setting a timer to give a fixed frequency interupt. 62.5 usec is the default.
Compare mode to OCRA is used to generate the interupt.
OCRR1A default set is 125. So we could tune the bit rate using a set of OCR1A or OSCCAL.
The problem was that it was not a fixed frequency error. The transmission would work fine for many hours
and then have dropouts for an hour. It seemed to be ambient temperature related.
The frequency is temperature related as per Figure 22-41 of the data sheet.
With my change of ambient we should only have a change from 7.93 to 7.95 Mhz.
This should not be enough to cause problems to VirtualWire unless the origional TX frequency is
on the edge of the default 62.5 usec value.
When I changed the transmitter to timer2 with 62.5 usec interupts the problem was solved.
Having solved my problem I did not chase down any further answers like provided in your reply.
Cheers.