From experience, my PEI sheet did not stay flat after multiple use, this may be causing the problems you are facing. It could be the adhesive is failing on different locations. 

As you mention, a new sheet of PEI may work for you; indeed the glass will not be necessary then, glueing it directly onto the aluminium bed would be enough provided that the metal bed is straight. Note that glass is straight as a result of the production process to make glass, so this can always be used. To remove the PEI sheet, a sticky glue residue is left behind on the build plate, please read ["How to clean up my build plate for a new build surface?"](/q/7960) for more information. It is probably easier to remove glue residue from the glass than from the heated bed as that is usually bolted and wired to the printer, but a wet cloth works fine. Personally, I print on glass or directly on the heated bed using an adhesion spray. You do not require the use of a PEI sheet.

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*Note that printing PETG filament is not recommended without glue stick or any other bonding agent like hairspray. This is not to get it to stick, but to get the print loose from the build plate, PETG fuses to PEI...*