I've heard both good and bad about about zapping, but as far as I understand, zapping basically can prolong battery life a little, but when it's time to use it, there aint much back anyway.
However, I dont know if that's the case with your packs.
What I do know for sure, is that discharging down to 0,9 volt pr. cell - preferable with an active equalizer - will help a lot. Afterwards, give them a complete charge.
A tray usually takes the cells all the way down to zero volt. This increases the output voltage, when the pack is in charged state. But it's also known for decreasing capasity. Since you're using a tray, this is probably a part of your problem. Taking cells down to zero volts usually introduce some false peaks, which might be your main problem.
Another issue might be your power supply or charger, which can cause false peaks.
Another source to the problem: Storing.
Please tell how the packs perform on the track, mainly about punch, but also about runtime. Also, is your packs sligthly warm after a complete charge? This can help diagnosticing the problem.
Schumacher Mi5 - Sanwa MT-4 / RX-462 / KO Propo PDS-2123 - Nosram 5600 X-treme Saddle Pack - LRP SPX Zero/X12 17,5 - Nosram Stealth Touch Evolution - MAAS SPS 9250