defender power problem

[offroader]

we have a defender 90 (1990) and we are experiancing problems with the power it seems to run ok for a while then seems to have a acceleration burst then back to slow again someone suggested it could be something to do with the exhaust has anyone out there got any ideas please

 

[joey]

More info would be helpful.... maintenance history? Last time plugs, wires were changed... I presume this is gasoline.... since you didn't say.