Strange shift problems

I have a '98 Sebring coupe with 155,000 mi., 2.5 V6 and the A604 - still original and "unopened", except for dealer fluid service. After about 90,000 miles or so, I noticed a few simple actions that can wreak havoc on my transmission:

With everything warmed up, accelerating from a standstill at low throttle (<2500rpm) the tranny shifts 1st to 2nd fine, accelerates some more then throws it violently into 3rd, sometimes making a "thud" noise which I imagine are the engine mounts clamping on for dear life...

Less frequently, the EXACT same thing occurs lightly accelerating out of a corner when the transmission goes from 3rd back to 2nd.

If I accelerate harder, the shifts are silky smooth, all the time - even at WOT.

From what I've read and been told, the Solenoid Pack is often to blame for simple shift issues - which is still the original unit... As far as I know the TCM was flashed at last tranny service.. but that's hard to prove. I would love to keep the car for another 155,000 miles but if I'm about to lose some gears or something I'd rather dodge the bullet now.

Any input as to wether to go ahead with a Sol-Pack change or a TCM flash would be greatly appreciated. Any advice as to the possible upcoming failure of my transmission is also appreciated, although not as much. :frown:

Thanks a bunch!