hesitation problem egr? pls help

I am having a big issue with my 91 ex coupe auto. I have been experiencing some serious hesitation in the rpm range 1500-3000 where the car feels like it is fighting to pick up speed and backfiring even though I have a new muffler high flow cat, new o2 sensor, new distributor, checked ignition timing etc.,. I was told it could be the egr valve so I drilled out my egr ports and cleaned them out well and also replaced my egr valve last year and it was cleaned b/4 installed. I think the egr is good b/c I get no codes and the diaphram holds vaccum when I place my finger on the line and press the diaphram up it stays there. I am sure there are no codes. After I cleaned the egr ports the car ran fine for a couple of days and the problem came back again. What else can it be? I have already tuned up my car--doing everything. any suggestions would give me an idea? I tried driving the car w/o the vaccum line connected to the egr valve but they sucks b/c I waste more gas, the check engine will then come on and the hesistation is still there.

Backfiring???? Have you done a compression check? How about valves? How many miles on the engine?

I did not do a compression test but the motor does have alot of miles 180k, this problem also starting happening after I changed the oil last month and I never used 5w30 which is the recommend weight until now. I was always using 10w40 but I don't know if that mattered with the compression that much

I would not be using oil any lighter then 10-30. The backfiring problem is interesting. Possibly leaking valves, distributor problems. I think a compression check is in order. You might pull the distributor cap and check the inside of the cap for containimatio nnmtairooigjtpu mou ou

Also check for stored ECU codes. It may give a clue.