i have a 97 4.0 se and sometimes when i put it on reverse i hear a "clunk noise" ,, and also when im driving it on cruise and hit the brake it feels like there's no power even when i floor the gas pedal, it feels like the auto transmission is not down shifting .....,,,

 

Have you tired to manually down shift it? If your tourqe converter is going out it could make noises and you will loose fuild pressure which would effect the transmission. My 95 40 SE's converter locked up and the shaft from the tranny stripped out the converter which broke my flywheel, it made a BIG bang and then I was stuck. Before it went I noticed a vibration just before it would shift into 2nd.

 

I think you have 2 issues going on. Rovers are know to have a little slop in the power train and will klunk between drive and reverse. You can help it a little by having the slip joints on both drive shafts lubed, do your U-joint while under there.
As for the lack of power, do you ever seen a check engine light on?
As already mentioned try down shifting manually and see if there is a change.
How long has it been since you serviced the Tranny?
Mike