Sounds like a very dodge install....
I can understand the high idle or the low power but I don't understand both! Take it back double time and stand on their necks until it's sorted.
How many miles on the clock? Belts need to be inspected regularly and replaced periodically; overhead cam timing belts every 60,000 miles. I suspect that the cam belt may be trouble.
<span style='color:blue'>"So my BM makes me a flash bastard? Then I guess you're just keeping it real from the seat of your nova?"</span>
<span style='color:gray'>"...and the home of the brave..... is bravey 130bhp from a 7 litre engine? 100bhp/litre or don't bother building it!"</span>
<span style='color:red'>"Aaahh the UK; where every road is jammed, under construction, poorly maintained and unfairly policed, at least we don't get over-taxed for it........ HA HA HA!"</span>