nigel hunt

i have a 1995 vw caravelle gl with 137,000 miles on it . it is the 2.4l 5 cylinder desiel non turbo , it has lost power slowly over the last couple of years ,i put it down to the miles but have been told that this is not big miles for this vechicle , it has really slowed down in the last month ,so i changed the fuel filter it made no difference ,i tryed the air filter it was the same ,at the same time i noticed when putting coolant in that there was a big build up of pressure in the reservoir , and when the engine is running with the cap off the reservoir it rises up and then goes back down when some bubbles are released ,it has been sujested to me that it could be the cylinder head gasket but there is no trace of either oil in the water or water in the oil , it also starts no problem and idles as it should ,can anybody help with a few sujestions .