Ok, so I have a 1.8l 8v (petrol) in the mk1 and I get stupid amounts of slippage when shifting into second gear during hard acceleration, but no slippage at all when pulling in first. Also, if I shift and fully release the clutch before planting the throttle then I get no slippage either. I imagine this is the clutch that's causing the issue, though I wanted to hear your thoughts.

I am thinking either a) dif or b) clutch, though I don't know what the quality is like for either of these parts on the 1.8l 8v motors (can get engine codes if that helps identify anything).

Also, would the gear ratio in the diesel box be hurting my acceleration? Currently I get out to 80kph pushing 2nd to 6-6.5k rpm, but I get that slippage that takes about 3 seconds before I get proper power delivery again...