I assume you have checked the disc spring washers are correctly orientated (5 of them, alternating in orientation as per manual) ?
Other things -
check the diff balls are 3mm ones and not flat spotted (roll them around dry on a flat surface like a glass table top )
check the condition of the diff plates (17mm washers as described in the manual !) - give them a once over with 800grit wet n dry to rough them up a bit, or flip them over if they are grooved
have you got the 2mm spring washer on the centre 8mm hex screw ?
I don't think the spares bag for the centre diff is too expensive - part 9405432 - and has all the parts
http://vintage-tamiyaparts.com/CMS/p...roducts_id=202