Rover 200 intermittant engine problem

while i am driving i will get a loss of power no power in gears 2-5 (cant get above 30) drop it into 1st go gentle on the accelrator sounds like mis firing then all of a sudden vrummmmmm its ok not mis firing (the car has to hit around 4500 rpm ) this can be intermittant, however now it cuts out when idel there is a slight pause when turn engition to start sometimes a strugle, i have changed spark plugs ht leads and battery dizzy looks dry inside no debri silver contacts(no corrosion)changed the black box by the ecu(silver box)(car worked great for a week and is now having these problems again,sometimes sounds and feels like a desile tractor engine rocks so hard i think its going to break the exhaust... any ideas or advice would be great, and thanks for taking the time for reading this.