If it was fine before you changed the mounts, I'd suggest that changing the tranny position is putting the shifter cable under tension. Result being you may be selecting 'half gears'. There is an adjustment at the tranny end. I'd start by releasing the inner cable (use two spanners or you will bend the inner). Then move the lever by hand so you know its fully forward (park). Test if it holds. Then move the lever backwards and forwards counting the 'clicks' so you know it has free travel and is selecting all gears easily. If you are going to test with the engine running, put somebody you trust in the car with their foot hard on the brake. You may have found the solution only to have the car run over you
Oct '00 E46 330i. Feb '92 525i (departed)