RBC Capital Markets has blamed management changes for its decision to downgrade electricity generator rental group Aggreko to sector perform from outperform.RBC said an analyst trip last week demonstrated the strength of Aggreko's market positions, its growth potential and its differentiation against rivals.But the broker added that the event failed to produce any new information and it believed the valuation of the company was now up with events.RBC added that it saw recent management changes as introducing some uncertainty, saying it had expected temporary Chief Executive Angus Cockburn to take the role permanently rather than Chris Weston."There remains uncertainty as to the timing of the announced changes, and witha new Chief Executive there is now some uncertainty about strategy going forward," RBC added.Shares closed 17p or 1% down at 1640p in London.PW