The fuel vent on my 175 exits the boat in the starboard bow just below the rub rail. I would check there, remove the vent fitting from the line, inspect it and blow air into it and see if you get much resistance. It's really weird but I've had debris get stuck in the external vent, I've heard others say bugs make nests in there. Really bizarre but that could definitely be it. You can also test if this is the issue by removing your fuel fill cap while underway and see if it solves you issue. If it does, you know venting is your problem.
The anti siphon valve in my tank (I don't know anything about your fuel tank, post some pics) can be accessed by removing the pie plate under your feet at the helm, in the deck. That's where fuel pickup and vent lines exit the tank. You can remove both lines and inspect that. The anti siphon valve is outside the tank where the fuel pickup meets the line that goes from tank to fuel separator.
Check fuel venting and knock that off your list. From there I'd start checking all lines, fuel pickup, anti siphon. Your screen at the fuel pickup could be gunked up. Your fuel lines could be deteriorating. I've had all of these things happen.
