Where
exactly did you tee off for your hot supply ?
I'm wondering if it's a bit high, and under certain conditions (someone runs the kitchen tap downstairs ?) the level in the vent is dropping enough to allow air into your shower feed.
You now have some air in the pipework, and the limited head you have available has to push that air downhill. Or put another way, where you should have a column of water pushing down through the shower, you have some air that isn't as heavy by a long way.
Lowering the shower head gives a bit more head, creates some flow, and I'm guessing you get a bit of air out as well as water. Once you've cleared the air, it runs normally.
Had a similar problem working in my mates bungalow. The hot supply for the kitchen tap came down from the loft to floor level, then back up to the tap. Having drained the pipework to do some work, there wasn;t enough head to push the air downhill while pushing the water up the pipe to the tap. I had to suck on the tap to get it going again !
That was one of the things I altered - lifted the pipework up a bit ! Then he went and ignored my advice (yet again) and bought a tap that looks nice but it totally unsuitable for the low pressure - so he's having a pump in the cupboard under the sink to make the tap work