Thanks both. I think you maybe right, its a nature of the engine. When it was released Tamiya were all up in arms about what they called a FRAT engine, Foward and Reverse Automatic Transmission.
"
2-Speed Automatic Transmission
Many features of the Terra Crusher give this machine superiority over its rivals. One feature that needs particular highlighting is the Terra Crusher's 2-speed FRAT(forward and reverse automatic transmission). FRAT enables the Terra Crusher to adjust to a variety of terrain by switching to low and high gears when needed to effectively maximize control. FRAT also realizes smooth, controlled reverse driving!"
I think part of this 'smooth' process must be something to do with the gears engaging and disengaging to get things moving.
Its only got one throttle servo, the single trigger does forward/reverse/brake seamlessly (in theory!). I'll have a look at the set up, see if there is any movement where there shouldn't be, I've found the manual just now, so going to use that to reset it all back to scratch!