My nephew called to ask my opinion of the diagnosis a mechanic gave him on his 98 Dakota 3.9l v6 4x4 auto. The vehicle runs but stalls is hard to start and generally is lacking power.

The mechanic is telling him the computer is going bad because of there are the (4) codes mentioned in the subject line. I looked them up and here are the definitions;

P0320 - Ignition/Distributor Engine Speed Input Circuit Malfunction
P0351 - Ignition Coil A Primary/Secondary Circuit Malfunction
P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
P0545 - A/C Clutch Relay Circuit Problem detected in air conditioning in clutch relay control circuit.

Seems to me the P0545 is unrelated and I don't know if he has any prior to this AC issues. Also seems the P0420 Catalyst code may be a symptom of the first two.

Does anyone have experience with the P0320 or P0351 codes. Is this something simple like a bad pickup in the sensor or could it truly be the computer going bad?

Time for a new mechanic.

My gut reaction is the cat is plugged, causing it to stall and then it is flagging the other codes. Hard to tell from this end of the keyboard, I won't throw the mechanic under the bus without knowing how much testing has been done thus far.
That code sequence doesn't scream PCM failure, but the P0351 could be a faulty driver in the PCM, even as the result of a coil that is shorting out on the primary circuit.
A resulting missfire can plug the cat, even without throwing missfire code(s).
The A/C code is not likely to be relevant, but I have seen stranger things.
The P0320 is odd, usually there is a cam or crank sensor reference and if that is the case, just replace both of them. They are cheap and the PCM does a lousy job of determing which is at fault, DAMHIK
HTH, Steve

Usually the p0320 is a crank reference not being seen by the pcm. I would put a new sensor in and a pickup while you're at it. The p0351 is lkely to be a coil issue, hopefully not a pcm. I would hang a crank sensor on it and a pickup. If the cat IS plugged, you can take one of the upstream O2 sensors out, leave plugged in, just take it out of the pipe and tie it up out of the way, and see how she runs. If it doesn't stall then put the sensor back in and drive it again, if it stalls with the O2 in and not with it out, you know you have a blockage in the exhaust, either at the cat or muffler.

I got a message from another forum from a mopar tech that had seen the P0320, P0351, P0545 combo twice before and it was the PCM.

Not sure where to start with the nephew yet. Rockauto sells remanufactured pcms for a little over $200. Trying to get my son to grab the mopar DRB tool from the dealership to see if we can see something more than a standard scan tool is showing.

Carquest, and Napa have the pcm's with really good waranties for right around that price...

I had checked NAPAONLINE and didn't see them listed but I will check a store. No Carquests near by where I live.