My orion 1.6 efi CVH has recently started doing this... between 1500 & 2300 rpm it hesitates like feck, but then its smooth from 2300rpm upwards.
When giving it a thrape, mine wont rev above 5000rpm now either. I'm guessing the camshaft is worn / wearing away, as mines been a gradual decline over the last few months.
I'd go for plugs / leads, but if they dont cure it then more than likely the cam's worn on it.