I might be barking up the wrong tree.. but worth a check.
Since doing your steam/PID upgrade would have required you to remove the brew boiler, my bet is that it's got an air pocket in the brew path somewhere..
Mechanically nearly identical to Synchronika, I've experienced this phenomenon after a rebuild.
If, when you raise the brew lever, accompanying a slight gargling and buzzing noise (which probably wasn't there before), you get an erratic fluttering of the pressure gauge, I might be able to help you.
2 possible culprits. One is air pocket somewhere between your water tank feed and group head. It could be anywhere. The other is water tank connection. Try in the order listed, to see if it fixes the issue (least hassle to most troublesome).
1) With one hand, press down on the white water tank, pretending it's not seated properly, and while pushing the tank down, bring up the Brew lever. If the connector on the water tank is not seated deeply enough, it seems to suck air a bit while drawing water from the tank, introducing air into the brew path. If this fixes it, your rebuild was inaccurate, maybe a misaligned top panel, not allowing the tank to sit low and deep enough.
2) If (1) didn't fix it, then it's an air pocket somewhere. Cold machine, loosen the mushroom slightly (maybe one rev.. Enough to make it leak water) bring up brew lever and run until a few seconds worth of water has run out from the mushroom, top of the group head.. Then while running the water, tighten and close the mushroom.
3) Hoping you didn't have to end up doing this, but you might need to push the air pocket out. You'll need to get a big plastic syringe and prime the pump. Disconnect one of the silicone tubes that comes out from the water tank. I recommend directly from the plastic nozzle of the tank itself.
Flip the Switch behind the drip tray forthe water supply from tank to line, because likely your water tank is remove at this point.
With a big plastic syringe filled full of water, connect it carefully to that silicone tube you just disconnected, being careful not to introduce large and new air pockets.
Then, in one smooth action, basically you need to start pushing the syringe down one handed (syringe won't budge, don't worry). Keep pushing on that syringe, while with the other hand, switch on the machine with the brew lever raised already. This activates the pump, and suddenly, it'll seem the syringe is being sucked. But the aim is to force feed the water, so this is when you start to push the syringe faster/harder.
This all goes very quickly. But be ready to turn the machine off before you run out of water in the syringe.
Turn the machine off, then carefully reattach that silicone piping to is rightful place without ceasing new air pockets.