GRE pppoe MTU

[Steve]

Hi all,
 
Looking to implement ospf over ipsec and need gre in the mix to make it work.
Even leaving out IPSEC I have erratic communication over the gre tunnels.
Pings always Ok but rdp or http traffic is hit and miss. I have been assuming
an MTU issue. I have max-mss set at 1440 on the pppoe interface and have
randomly tried different mtu settings on gre interface down to 1240 with no
luck.
 
Any thoughts would be appreciated.
 
Thanks