Its just as Ram says. It's the only loco I have with that worm gear style of mechanicals & the only loco with the problem. It wants to go faster & putting speed on it stops the surging. I've tried shimming anything & everything that shows some slack, mostly the fore & aft play of the motors drive shaft. Even the slightest play & it surges & when I've I shimmed all the play out it tends to lock up, requiring some quick forwarding & reversing to get it moving. I might have to persevere with Running Bear's suggestion & try to find the "sweet spot". Failing that, I'll just have to accept it as it's quirky nature, paint some GT stripes on it & just go fast!

I've recently gotten back into model trains, not having anything to do with it since I was a kid. My Mum found a few of my original HO locos & rolling stock including a Bachmann CP Rail GP40 that I originally received in 1975. Even though it's close to 35 years old, it's still in excellent condition since it didn't suit my childhood requirements, ie. going flat out on full throttle. Now my speed requirements have mellowed with age, I'm finding the only way that this unit will go down any sort of grade without a surge, pause, surge, pause etc motion is to go flat out again. It does this on it's own, with no load & I'm sure it's done it since new. I've stripped it down several times & lubricated all moving parts with a light oil, then a light grease & finally a heavier grease with no improvement. Different power supplies using straight DC or pulsed DC make no difference. Another post on this site had a similar problem that was solved with the removal of some capacitors, but it was a modern DCC type, & mine has no caps to remove. I've seen a "Life Like" brand model that looks very similar internally but has flywheels & wondered if a Life Like motor would be compatible & the flywheels solve the problem. It's a good looking loco with heaps of power & grip up hill but the surging means it's nearly useless when it comes to pulling any load as they'll derail & pile up behind.
I was hoping someone may have some suggestions for a remedy or an upgrade that may fix an otherwise great (in my very limited experience)
locomotive.
Thanks heaps for taking the time to read my problem,  Robert G.