mk II gearbox?

my mkII 8v recently started chugging when cruising in 5th gear. it stops when i accelerate hard or lift off. i got the gear box reconditioned which didn't help i then changed the drive shafts but it's still doing it. any ideas how i can fix this as it's really annoying me? have i been screwed by the gear box man??

 

I think you have spent a lot more money than you needed to!

 

i know that lol. so how do i fix it b4 it breaks me [:^(]