Operations has had problems with the current actuated startup bypass valves.  The fluid in this aplication is Blend Oil.  There is currently a 30s shutoff actuator on a 1/4 turn ball valve.  Because only about 1/8 of the turn is actually doing something, they find it shuts off the flow too quickly.

The typical ball valve will get pretty much full flow at 5-10% open and no slow down in flowrate until they reach 90-95% closure.  If you're getting as much trouble as you seem to be having, I would suggest you look into a different port shape.  Different port shapes may allow a smoother flow control in your application.  You must find a valve with a Cv coefficient vs percent travel curve that is not allowing so much flow at low % opens and has a slower and more evenly proportioned flow adjustment on shutoff.  

It is possible that a V port ball will work, however depending on the severity of your problem, you may need a globe-type control valve that can be accurately adjusted through most all percent of accuator travel.  There are several common flow to travel curves for various valve types and port styles that may allow you to nicely proportion the % open and % flow to solve your particular problem.  You just have to get the valve Cv-%travel characteristics off the vendor's websites, calculate the flow at your particular start-up and shut-down pressure increases and decreases and find a good match.

It does sound like a linerarity problem, you are going from basically full-flow to shut in the last second or so of your 30 second travel.  My first choice for a solution is the same as yours, I'd put in a valve with more linear valve-postition vs. flow-rate curve and my preference would be for a V-Ball.  I'm not sure what your valve salesman was thinking of--he's out of stock on V-Balls, maybe?

Keep in mind that no proper throttle valve will give you a reliable shut-off seal.  For a bypass I'd back up a V-ball with a ball valve that is normally shut during steady-state

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