As I posted earlier, I have a similar problem with one I bought from Jumblies.  I only paid £63, and Jumblies can only offer a refund not a swap, so I am determined to get to the bottom of the slow running problem.

If I remove the chassis keep-plate and the body there is nothing obviously wrong with any of the parts and the pickups are all good, and there is no over-greasing. With the keep-plate back in place, but with coupling rods removed, and the loco on the track, the motor will turn over the rear wheels at a *very* slow speed with no hesitation - about 1 rev of the motor per second. The loco won’t move but the rear wheels turn on the spot perfectly. So there is nothing wrong with motor, gears or pickups. It’s only with the rods back on that the sticking occurs, and it is only in one spot per revolution. My suspicion is either that the wheels are not quite quartered right or that the coupling rods are not quite drilled right. The centre hole in the rods is slightly oval but I’m guessing that is deliberate.

I spoke to Oxford Rails' technical dept on the phone, but they are not aware of any specific problem with the Dean Goods. I have emailed them this morning asking if they can send a couple of sets of coupling rods for me to try. If that doesn't work I'll start opening out the holes in one of the sets.