2002 Vauxhall Astra engine cuts out and under revving

For the last month or so I've been experiencing difficulties with my car. It started with the oil light flashing on and off, so topped up the oil. All was fine for a couple of weeks, then the engine electronic light came on. Restarted the engine, that went off and oil light re-appeared. Switched the engine off, all was fine for a few days. Then suddenly the car engine started to under rev occasionally. Now the car under revs most of the time, usually after the first 20 miles of driving it. In the last few days, the car engine has suddenly cut out when braking, then having trouble restarting. The engine management light has also appeared for 60 miles, now goner off and replaced by the engine electronic light again, but no more under / over revving of the engine!
Can anyone help...please? It's going on the diagnostic tomorrow, however Vauxhall's sometimes spurt out the wrong diagnosis, which worries me considering the problems are not consistent but could be linked.