That's a strange one, (tobytronic's experience) since I've noted tight clearances to actually make a fair difference to cold starting. Perhaps it's down to running on 100% veg oil so it's more sensitive to the slight loss of compression when stone cold, but either way I've never known very slightly large clearances to affect either cold starting or general performance, nor create an intermittent problem like above.
To be honest it does sound more likely to be a fuel issue, or possibly (one I experienced on the silver car last year & only noticed while revving from under the bonnet!) something in the intake isn't up to the task & is collapsing under the draw from the turbo. Either a collapsing pipe between the filter/turbo intake or even the filter itself depending what type you have. When it loses power do you notice any change in smoke, more or less, different colour etc? EGT gauge would probably help determine more if it's a fuelling issue or air restriction. (EGT suddenly less than what you'd expect = fuel issue, sudden increase from what it'd normally be would= lack of air)
One other thought, does your engine have an EGR valve still installed? could possibly be getting sticky or weak & allowing boost to bypass straight to the exhaust side.
To be honest it does sound more likely to be a fuel issue, or possibly (one I experienced on the silver car last year & only noticed while revving from under the bonnet!) something in the intake isn't up to the task & is collapsing under the draw from the turbo. Either a collapsing pipe between the filter/turbo intake or even the filter itself depending what type you have. When it loses power do you notice any change in smoke, more or less, different colour etc? EGT gauge would probably help determine more if it's a fuelling issue or air restriction. (EGT suddenly less than what you'd expect = fuel issue, sudden increase from what it'd normally be would= lack of air)
One other thought, does your engine have an EGR valve still installed? could possibly be getting sticky or weak & allowing boost to bypass straight to the exhaust side.