We picked this 98 Convertible Sebring up very cheap for our daughter. It has 57,000 original miles on it, was owned by an elderly "snow bird" and kept garaged when not being used 6 months out of the year. It is in spectacular shape except for some drive-ability problems.

When we got it, it wouldn't hardly run at all. It would start, and chug about 5 mph but that was it. Give it any gas and it bogged down. I thought bad fuel, tune-up etc. Ended up draining tank, changing the plugs - wires - distributor and IAC. Wife said it behaved much better after that so I left it alone..

I drove it the other day, and found it seriously lacking any power at all. It takes forever to get up to 55 mph merging onto the freeway. Sounds horrible when stomp on it, like exhaust leak in engine compartment but can't find one. RPM's get up to around 4500 max but tranny feels like it is slipping badly. Fluid looks fine though, not burnt.

It doesn't want to reliably auto shift on hard acceleration either, have to let off gas to get it to up shift. At 70 mph it starts exhibiting what at first seems like a high rpm miss, but the tach surges instead of dropping off. Then the CEL comes on.

It threw these codes today: P0172, P0420, P1391. Now I know of a bunch of things that "could be wrong" but don't like blowing cash on parts that are not failing. It could be a plugging up cat, or bad O2 sensor, or fuel pressure / filter issue, might also be a slipped timing belt but don't know enough about the 2.5L V6 to know if it would run at all if it slipped a tooth.

So, where do I start with this to get to the root cause?

 

Welcome to the forum.
There may be more than one "root cause" here so I don't expect there will be a magic bullet that will solve all your problems with one tiny fix.
The 0172 code and the 0420 code both point to the catalytic converter on the rear bank of cylinders. The loud exhaust noise that you can't see might be another indicator. It would probably we worth the effort to get the car up on a hoist and carefully inspect that cat and pipe for damage/leaks/etc. An internal break in the cat can be noisy but not visible from the outside.
The 1391 code points to either the crankshaft or camshaft position sensor. Since you have replaced the distributor, and that includes the cam position sensor, it might be time to look at the crankshaft position sensor. Be advised, though, that aftermarket distributors have been known to have issues right out of the box, so you can't rule that out either.
As for the timing belt, it could be 14 years old, and you don't want to wait for it to break before replacing it.
The Chrysler transmission is very sensitive to bad quality trans fluid. That fluid could also be 14 years old and even if it looks good, it may be due for replacement. That's probably the cheapest thing you can do for the trans that might have a chance of fixing it, so you probably will want to do that before any major rebuild.
There may be some codes in there that can be reached by a more sophisticated reader. You might want to have the car scanned using the Chrysler DRB III scanner to see what lurks inside. A dealer will be able to do that, as might a high quality independent repair facility.
There was a reason you were able to get that car so cheaply in the first place, now it's time to spend some money to increase it's performance and value.
Keep us posted on your progress.

 

I replaced the crank position sensor today. Drove it around for an hour stop and go city driving, thought maybe it was finally fixed. Had good acceleration from stop, shifted thru the gears nicely, etc. Then at idle the oil light started flickering and came on, and the idle occasionally slightly bogged. It has plenty of oil.

I decided to see how it would drive on the freeway. Pulled hard thru the gears on the on ramp, got up to 70 mph fine, then BAM.......... CEL came on again. So I decided to keep it on the freeway for awhile and see if the tranny issues came back. For 20 miles all was well.

Unless I tried to use the cruise control. It worked, but hitting the resume button if going 70 mph or faster shut the cruise off? Is there some kind of limp mode I am hitting here?

Managed to maintain 80mph for 15 miles until got off and headed to Autozone to check the CEL code. It was 1391 again, momentary loss of cam or crank signal. Driving back on freeway, all the tranny issues came back, the tach going nuts, the surging and slipping.

Why is the 1391 code only getting thrown under hard acceleration and higher rpm's?