Well it seems you removed event 9 and 10 from the auto-runner template which as stated in the above comment are for:
"Due to the acceleration of the platform movement, we need to do some tweaks to make sure the player doesn't gradually get pushed off to the left of the screen over time."
Adding them or something similar would solve your issue..
While upping the bullet speed also make sure to up the player speed, for else indeed the problem will get worse..