my disco 2 has got a bad miss fire ! it starts normal, runs & revs normal
for about 10 seconds & then runs lumpy ! what could it be

 

A number of things.

Clogged injector(s)
Bad plug wires
dirty trottle body
Idle control valve
vacuume leak
MAF sensor
and on and on....

Is the check enging light on? Go and have the trouble code read at autozone for free or buy or borrow a code reader and get back to us with a little more information.

 

If there are no codes, I'd check fuel pressure.
pressure should be 50-52 psi regulated by the fuel pressure regulator integral to the fuel pump.
Test port is located above the coil packs between the rear of the engine and firewall.