OK, in a way that doesn’t make any sense, the lid being airtight, or just loosely on should not make a difference to the pressure. It’s also intermittent (seemingly). To me, it sounds like the pump is either:
- faulty
- has a restricted inlet flow.
The first requires a new pump, very easy to fit and they can be bad from the factory, have a bit of debris inside them etc..
The second could have a problem anywhere in the inlet tubes all the way to the pump. Working from the tank it could be a blocked gauze filter, split in the tube, blockage in the tube, a kinked tube. The machine has 2 pipes out of the back, one leads to the inlet….see if you can eliminate areas of the tube as having any problems….
Then remove the case and check the tubes inside the machine are not kinked and check for blockages. If you can find none, then it must be the pump….from your answers (if correct).