Chocolate Gearbox?

[Guest rmilbank]

I recently put a different gearbox in my car as it was jumping out of 2nd alot. Whilst doing it i fitted R/J linkage and H/D engine mounts. Now i have a n even worse problem, whilst accelerating im fine. As soon as i cruise or come off the power it drops out of 3rd and 5th. I tryed the original mounts back on but its still doing it. Do you think the synchros are shot??