Good advice Iain,
Checking the line with the line hand will kick the fly over, I do it a lot.
Paul,
It sounds from what you say about the problem being mainly with smaller flies and not with bushier stuff that it is probably a leader thing and using a knotless tapered leader should sort it out. A 7wt flyline carries a lot energy, the straight 5lb leader is probably turning over so hard that the fly is recoiling back on its self, the energy in the line has to go somewhere. With large bushy fies, more energy is required to get them to turn due to them being more air resistant, a tapered leader will dissipate the energy and give a more controlled presentation with the small fly. You are also probably using too heavier line for the flies you are trying to cast. When deciding what line weight to fish, you should first consider the size of the flies you are casting and choose the line weight based upon that, small / light flies = light line.
Cheers
Ben