intermittant stall/rough idle

i have a 1990 Limina (my son's name for it) with a 3.1 mpfi.

usually starts fine. will idle for 3-5 min then stall or begin to idle very rough. when it stalls/idles rough the low coolent light flashes then the service engine soon light comes on.....all of the relays in the engine compartment except the fuel begin to chatter, the fan (sometimes both fans) come on and the idle air valve cycles (probably why it wont idle). the only code i have gotten is 43.

if i turn the key to run without starting the engine and wait 3-5 min the low coolent light flashes then the service engine soon light comes on.....all of the relays in the engine compartment except the fuel begin to chatter , the fan (sometimes both fans) come on and the idle air valve cycles.

when i drive it will run fine for 5-10 min then begin to run rough and surge. if i let off the gas or stop it will die. if i wait 1-2 min and restart it usualy will run fine for 5-10 min. sometimes the service engine light comes on and stays on - when that happens the car runs fine but smells "rich" and idles high (1500 maybe, no tach).

i tested and replaced the coolent temp sensors and the IAT, the ignition coils and CPS test good (using hanes as a guide).

im thinking that the ECM/PCM or PROM has gone south but dont want to buy a new one to troubleshoot - they are fairly expensive and i like to know what i am replacing is bad.

does anyone have a test for the ECM/PCM?

any help will be appreciated

Since you did all the other stuff, next step is:
What is code 43? Have you added coolant, if so, do you keep on adding coolant. You could have a bad head gasket or intake gasket. Is there any oil in the coolant?? Since you have Haynes(for specs) check the compression in each cylinder. How is your fuel pressure you should check that as well. I assume wour coils,plugs,wires are up to date.
Do the pressure checks and find out what code 43 means. Get back to us.

the code is indicating an ignition fault. coolent level is good, no leaks, no oil in coolent or coolent in oil. haven't checked compression or fuel pressure yet, mostly because this seems to be an electronic problem with the symptoms i get with the engine not running. ignition coils test good. new plugs, will be new wires this weekend.

i forgot to mention earlier, when i am driving and it acts up the speedometer drops to zero....i think the speedometer gets its input from the ECM/PCM.

I forgot to ask your battery is good,right? You should have a 12.6 voltsignition off, and around 14.3 volts on. I know wires should be cheap, but double check the resistance of your wires. Check your haynes for specs.

batt reads 12.6 engine off, 14.5 engine ideling.

i have been through the wiring checking for any problems with any thing going to the sensors....it all looks good, not finding any shorts/opens. the car seems to run best when its cold. i drove it last night when the temp was about 20 deg and it ran pretty good. this morning i had a code 51 (MEM/CAL or PROM). like all of the indications it is intermittant....started it, idled for a couple of minutes, shut off and checked for codes and the SES wont even flash??!!!!!

any suggestions?