Rough idling when hot

I have a 1993 Mazda B2600i automatic transmission with 180,000 miles. The vehicle runs great when it is started after it has set for an hour or more and will run until you turn it off or runs out of gas. However, if it is run and stopped (say to get gas or short trip to the store) it won't idle correctly.

It will generally start (not always) but it continuously needs to have the throttle applied or it will stall. The idle is very rough and very low.
The problem has ocurred since buying the vehicle with 50,000 miles on it, but it was generally only a problem after making several short trips in a short amount of time and it would go away after it was driven for 20-30 minutes continuously (it would idle fine and would not need to keep the throttle on). I took it to the a dealer at this time and they told me to tap some part on the engine (that something was sticking) and that would cure it and it did then, but not now.

Now if the vehicle doesn't cool off (I guess) enough before being started again it won't stay running without continuously giving it gas and runs very rough while driving. Gas consumption is also considerably more during this time. If it running correctly I average around 28mpg on the highway (which is most of my driving) if it is not the mpg drops to around 13-17mpg.
The only error lights that come is the light for the low oil pressure. Oil level is correct, I check it every time that it happens and before I take a trip. Current there is Castrol 10W-30 in it.
The issue got bad enough in 2005 that it would lose acceleration and going up hills sometimes required pulling over stopping (not shutting off) and starting out in first gear again. When it got to this point I took it to a garage and the code said it need a new computer, so it got new computer.
The O2 sensor has never been replaced, I was going to have it done, but the dealer said that they only replaced them if they were bad and it wasn't at that time (it had the problem then).

Also when it is running rough it has a problem downshifting. I noticed the last time I was going up a steep hill that it acted like there wasn't a third gear it has a four speed transmission.

I have done the normal maintenance (i.E. Oil, coolant, ATF) at the scheduled intervals and I am not a hard driver, very conservative.