'P' Reg 3.9 v8i Petrol. How could I determine if it was the running gear??

Speed related...any ideas???

My disco seems to miss slighty on one cylinder....but really only when i'm doing between 60-70mph. It just had a service and I have replaced all the spark plugs and HT leads....any ideas?????

Well that makes feel better as I am getting it look at by a landrover mechanic, then if all is well it will be lpg converted. Thanks forthe reply

Just bought a V8i 'P' Reg. In the morning when I start it, it sounds like it is missing very very slightly and it does put out steam with a few little droplets of water.....could this be the head gasket going...or could it be something else before i get too depressed.....

For head gasket problems....has anybody used STEEL SEAL???...

Just bought a P reg Discovery (petrol v8i):):) Just had it serviced.....a few things Maybe has a problem...Cold morinings, steam comes out the exhaust and the engine misses a little until its warmed up a little (few moments)...could this be the head gasket about to go....or is it normal.... HELP...