"when changing gear. Instead of the revs dropping , it stays at whatever rpm it was before the change. " is that when changing down the revs stay the same, changing up they will stay the same if in the same rpm range
"clutch depressed (in gear), foot off the accelerator and the car just rolling along, the RPMs rises suddenly to about 2500 then drops down to idle. A few seconds later it does this again. As long as the above conditions are in force it will do this until I put the gear in neutral.or stop completely. "
So you have the car in gear and clutch pushed in .. so the clutch is engaging again, lower clutch fluid/pressure/spring