Grubenstein

All sorted now. Updated to 2.9.460, 
Actioned some of the excellent points above
Most of my settings were on low anyway.
But getting 72-36 fps at the start of mission 1 of F16 Red Flag Campaign and 72fps FA18 BFM campaign
Thanks for everyone's help

Soved

Updated DCS and re-flew issue sorted. May it was that or I need to get to the IP BEFORE radio call

You comments gave me a clue to the problem. You said I was slewing the radar cursor (I wasn't). But then that made me remember that as I have the cursor on an analogue stick on my throttle. Sometime the radar slews across to the left on its own. So I cleared the axis assignment and used the digital stick and problem solved. 
As the analogue stick was slightly on the move it was preventing me from changing modes
 
Thanks everyone for your help mystery SOLVED!!!

Hi Flappie. F16 but don't worry sorted it. I was too slow and the bomb fall line was right at the edge of the circle that appears prior to release. Sped up to 400 and releases ok.