That thread over at Endless Sphere was started early '09, and there was only one person there which had been sent some (obvious) used batteries; probably neglect from one employee... The rest of the thread are more of an apology from Yao Yuan regarding the wrong track the company had taken during his absence. (and a promise to be better

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There has been some cases of bad cells in some of the GM packs last year in what i can see from the GM forum, but i think GM resolved most, if not all those cases...
I'm not trying to defend GM, but i myself - and my brother have 3 36V16Ah packs from GM, and all of them have performed perfectly the last 2-3 years.

That said; one of the FET's blew in my 'magic' controller (letting the magic smoke out), but being an electronic nut, i replaced the fet easily. After looking at the problem from all angles, i concluded the fet was from a weak batch...
(edit)
GM are still struggling when it comes to customer relations, but there are now some trustworthy distributors one can rely on.

(/edit)
What kind of controller are you using? I have only experience with GM's Magic Controller. Sounds like your pedlec works correctly; the motor should work only when pedalling, and not above 25km/h (EU regulation). That said, on GM's controller, one can turn on/off the pedlec mode, and when on, the throttle are disabled (no boost function), and when off, the throttle are just that; a throttle (and pedlec are disabled).

My throttle works exactly the same way as on a motorbike; 1/3 throttle = 1/3 motorspeed etc. and for coasting, i just press the Cruise control, and the controller locks the (motor)speed at the current speed....

(sadly, not the bike speed)
Due to the hall sensors for each phase in the motor, the gm motor/controller knows when the motor has a certain speed that matches the throttle position.
I'm guessing your speed/power drain problem are due to the SLA's inability to deliver enough continous power over time.