when i accelerate in 2nd or 3rd the engine seems to slip into neutral then rev high and release, this happens every hundred yards or so until i get into 4th. any ideas?
quite correct, engine stripped and cruciform is knackered.
Its the cruciform!!
almost always goes between 2 and 3rd when putting most power on!
replace it as it will only get worse[:(]
but also check the cables are not too tight on selector, and the pin in the selector locating arm(tiny pin which can be unscrewed, to remove the arm with the wheel holding the gear slecting cog in place!!!)
as this always wears, meaning it will not hold the gears, replacing it will stop the slop! or carfully drill insert a small tube and that will solve the problem for good(a very bad design, as it pivots on two small edges meaning it will wear very quickly)
try taking the selector box off ,hold the arm inside and wiggle, to see if it moves with out the wheel moving meanig the pin is worn
also if using the older style gear box, replace with a newer style(flat cruciform) as the layshaft has cutouts which seem to hold the gears better!