I have a Lan to Lan VPN betweek head office on a 2820n and another office on a 2710. They can ping each others gateways with very few if any drop of pockets, but soon as I try to ping a PC in the network it drops lots of packets probembly about 40% making it pretty much unusable? any suggestions? changed the MTU to 1400 seem to make it worse!

Did you change the MTU of the WAN connection or the VPN? There's telnet commands for both but IPSec usually goes with an MTU of 1360, so probably setting an MTU of 1400 is hurting it.
Unless your WAN connection requires a smaller MTU, I suggest using an MTU of 1458 for the WAN connections.

Also is it an IPSec LAN to LAN or a PPTP LAN to LAN? IPSec is the faster of the two.