odd engine behaviour

[wargriff]

I wonder if anyone can help,

when running along happily my car suddenly decides to make a deep throated engine/exhaust note, it then struggles to accelerate and will continue like this for a short period, it ususally happens in 1st and 2nd gear, but had done it once in 5th.

does anyone have any thoughts to what this could be as it is very odd ?

any comments welcome.

[5.0ltr]

Similar to mine last year which was the thottle potentiometer. 1 ECU reset later and has not done it since, my 2p worth anyway.