914 completes runup, but then occasionally gets extremely rough on takeoff attempt

Small amounts of water in the gas?

Good suggestion, but I check the water separator sump/fuel filter regularly and have never found even a drop of water in it.

Thanks!

Debris in the float chamber that is drawn into the main jet under conditions of high fuel demand?

I like Kevin's suggestion - problem most likely in float bowls but could also be elsewhere in fuel supply.

Visual check of filters does not always give accurate information (some debris can be almost transparent). Remove filters - clean and or replace.

Using compressed air, blow out fuel lines.

Don't forget to check fuel tank breather.

Check performance of fuel boost pump.

Thanks, All good suggestions! 

I figured it is probably in the fuel system. Fuel pressure goes up properly with turbo boost, but may have something that got by the filters so I will check out the fuel lines and bowl floats carefully. It's just a little baffling how it only happens randomly and only on the first flight of the day. Something in a bowl makes the most sense.

Thanks for all you suggestions and help!

I will reply back if I find something definitive. May be a month or so before I get to it.