Great to see you, Rob
As you mentioned the tires, maybe going a bit smaller with these?
http://store.rc4wd.com/Rock-Crusher-...res_p_730.html
Those are the popular 2.2" wheel size.
Next size up is Series 40 - with tire diameters of 200mm or more - so these would be too large.
Apart from that, maybe the traction could be improved a bit with more weight? I guess your wood construction is fairly lightweight?
If so, some people put lead or steel into the tires. I am a bit scared of lead's toxicity, so would look at different solutions first
Cheers
Jens