at 60 mph bongo seems to jerk as if changing up and down?

[beasd mr Friendee]

Anyone know what this might mean? is it normal. It always happens at 60mph but intermittantly not constantly, go under 60 and it stops happening. just feels as if the bongo decelarates for a split second but you haven't moved your foot, then a few seconds later the same.
a little while later same thing, can be seconds or minutes between occurances but this may be about speed fluctuation?
FYI we are proud owners of our first bongo, picked up from newquay mazdabongo company.
its a v6petrol with lpg conversion, s plate.
have read that lpg may switch to petrol if struggling for any reason so wondering is this a point at which the fuel is switching over then switching back, or is 60 mph just at a point where the gear box is changing up and down.
I'm assumeing there is some kind of fault other wise its a bit of a poor design flaw?
Anyone know anything or can suggest my next move?
Have three month warentee so don't want to miss opportunity to get a problem fixed as neither of us is any good with diy motoring ( have to follow instructions to put in oil!)

[francophile1947]

Don't know about the LPG bit, but the gearbox won't be trying to change gear at 60mph.