I just put a never installed heavy duty T5Z in my Mustang last month. It seemed fine for the first 600 miles or so. Last week I did a downshift from 3rd to 2nd at about 35 MPH and it jumped out of 2nd. Ever since then It will jump out of 2nd most of the time on a downshift, but only on a downshift. Even if you are pulling back on the shifter handle. When it jumps and you are holding back on the handle it grinds for a second, and seems to find a spot and goes back into second and then it wont jump back out till you downshift again. When you shift to second it feels solid like it is going all of the way in on either an up or downshift. If you upshift from 1st to 2nd it will stay in second even if you run up to 35 or so and push the clutch in and let the motor go back to idle and let the clutch back out quickly. Not so if you downshift from 3rd or 4th back to second. It never jumps out of 2nd on an upshift from 1st. It never jumps on a downshift if you apply power. Only if you are decelerating. I thought it might be the shift stops on the shifter so I loosened the 2nd gear stop till there was at least a 1/6th inch of clearance when you shift to second. Second and 4th share the same stop, but it wont jump out of 4th. Nothing changed. Remember.... this was a new transmission when I put it in a month ago, and was perfectly fine till last week. I have never abused the transmission....powershift etc... I have a new Pro 5.0 shifter I can put on it, but I don't think will make any difference. I am at a complete loss as to why this is happening. If anyone on the forum can help with this I will be very grateful.