93' Diamante hesitate to speed up from 30 miles

I bought a 93 Mits Diamnte (SOHC) from a friend of mine about 3 yrs ago with 70K miles. Now it has 100K and it has been a good car. Ever since I noticed (more during the winter) car has a hard down shift when I come to a sudden stop. I took it to a local transmission shop, and they couldn't find any problem with the tran, they replaced the engine mount (rear) but the hard shift is still there. I can live with that, but the major failure is:
Car hesitate to speed up from around 30 miles, even if I press the gas pedel to the floor, the car very slowly speed up. It accelarates really good in neutral. It is impossible to catch up an yellow light or even getting on to the freeway.
I have been working on this car, and I have done the following so far:
Replaced spark plugs & cable, distributor rotor, fuel filter, air filter, PCV, adjusted the ignition timing, checked & cleaned TPS, MAFS, changed the trans fluid&filter. I am running out of clues now other than taking in to the dealer, which costs much more than the value of the car.
Any suggestions are welcome,
Our Good Lord Bless you all for your support.
James

Mine did the same... I am trying the idle control valve and then the oxygen sensor... Found them on ebay.