First things first, don't buy any new parts yet.
Take off the grip frame, and check the valve jam nut. You don't have to pull the entire internals, just take off the frame, and using a small pick, see if you can move the jam nut. It should be rock solid.
If you aren't 100% sure the jam nut is tight by using the pick, remove the valve plug in the body of the gun, and try to rotate the valve body. Again, if you can move it at all, it's a serious problem.
If either part moves, pull the hammer, and tighten down the jam nut.
If the jam nut is loose, it can limit the hammer travel against the valve stem, causing low velocity. That's an issue, but the real problem is that it can destroy the threads in the body, and render the body of the gun useless.
For details, see the my old site in my signature, there's a walk through on installing a valve, which should cover you.
Once that is out of the way, you need to check the inline regulators output. You will need an ASA of some kind, and a gauge. Gas it up, and look for the pressure to creep up. If it does, rebuild the reg with a new reg seat.
You should also be able to adjust it from near zero to over 500 psi. If the reg can't do that, pull the reg apart, give it a cleaning, and rebuild it.
Once you have confirmed the mechanical aspects are working correctly, then you should try to sweet spot the inline.
There's an article on how to do it in the site in my signature. I wrote it years ago so I wouldn't have to keep typing it up in posts like this
Start with the IVG at the mid point of it's travel, and see what results you get. If the sweet spot is too high, then back out the IVG, and redo the sweet spot.
The sweet spot is a balancing point between the force of the hammer trying to open the valve, and the valve spring, and the valves resistance to opening because of air pressure.
You can easily control two of those variables, the main spring tension, and input pressure, so you will be working with both of those.
While doing this, I'd suggest turning up the LPR so the gun cocks with the IVG half way in, and leaving it there for now. You can turn the LPR down and smooth out the cycle once the gun is shooting at field speeds.
Focus on ONE THING AT A TIME. Get the gun shooting at speed, and running as well as it can with what's already in it, then you can think about things you want to improve in the performance of the gun later on down the road.