rharreMach1

So I was wondering if I could get some help from you guys. I have a 2003 Mach 1 with an Automatic trans. I bought the car a few years ago with a rebuilt trans that was beefed up with a shift kit, 4.10s in the rear and a Predator Programmer. Lately it drives fine and shifts through all the gears great but once I begin to brake from a speed of 40 up it downshifts hard(at higher speeds)and kicks between third and OD, once I get back on the throttle it shifts back into overdrive. I had my local mechanic look at it and replace a solenoid (what he thought might fix it) worked for awhile as I was really cautious when it first got fixed. Sure enough it came back and is still making me nervous. Anyone have some insight on this? I have very little transmission experience. Thanx. preciate it.