headgasket ?

Taynfin came up with the following;:

Head Gasket, leaking into cylinders and exiting via exhaust, so not really
noticeable elsewhere.

Yup most likely head gasket failure , you will probably find that the
gasket has gone between the water jacket holes and probably no2 or 3
cylinder, as the engine works hard and warms up the pressure rises in
the water jacket and is forced into the cylinder then out the exhaust
usually you can see this by alot more steam than normal coming out the
exhaust , needs doing asap or the bore will become pitted and if to
much escapes into the cylinder you could risk hydralic lock and wreck
the head . you can lessen the problem to get to a garage by running
without the radiator pressure cap fitted .....