vibration at 3000 rpm

My engine has developed a vibration between 3000 and 3750 rpm. It started at the weekend and is getting worse

It worse when cold, and worse on petrol

I have recently fitted a new rotor arm and vacuum advance, the vacuum advance unit appears to be upside down compared the old unit.

Any ideas what it could be?

Its a 1995 disco 3.9 v8 with lpg

for those of you that are interested, or have or will develop a similar problem. the fault this time was the viscous fan.

Both the viscous fan hub and and water pump bearing will cause a vib if faulty but if you think it's a mis-fire then the ignition/fuel system is the first/second port of call for looking at/testing.