Excellent Nemo!

Crystals can go either way, I'm unsure if using incorrect crystals can damage the boards. With the control lag, the only times I've had laggy controls is if either of the batteries are flat OR if the antenna wire is missing, broken or poorly soldered, or the tank is right on the transmitter's range limit. Check the wire, make sure its not crimped or damaged, and that its soldered on well. Another possible cuprit is the little washer soldered to the antenna wire end is oxidized or just not making good contact.

Always check the crystal sockets when changing crystals, they can easily be bent out slightly: one leg might sit in firmly, but the other just sits in mid-air, because the contacts in the plug are spaced too far apart [8D]