92 Eclipse with severe engine stumble

I have a 92 Mitsubishi Eclipse 4 cyl 1.8L with a SOHC, non-turbo, 5-speed manual. It has 165,000 miles on it and is well maintained. The car has developed a severe stumble when accelerating in 1st gear from a stop. It appears to be an lean air/fuel mix problem. I have cleaned the fuel injectors, and the fuel pressure regulator, replaced the PCV valve and fuel filter. I have also replaced plugs, wires, and distributor cap and rotor. I have also checked for obvious vacuum leaks. None of this maintenance has solved the stumble problem. The strange part is … if I start from stop in 2nd gear then there is no engine stumble. Car dives well after I get it started off the line. With this many miles on it I suspect there is some degree of blow-by but unsure how this would cause this 1st gear start-up stumble. I did not do anything with the mass air sensor or oxygen sensor since a failure in either of these would cause a rich air/fuel mix. The car does not have an EGR valve. Any ideas as to my next move?

Additional info / update … yesterday I spray cleaned the throttle body and added an additional bottle of STP fuel injector cleaner to the gas and 4 cyl Engine Renew additive to the oil. This morning, with 20 degree temps, I took a short trip and again experienced the engine stumble but after a few miles the problem disappeared. This has happened before … severe engine stumble that disappears only to return sometime down the road, usually after a high speed trip on the interstate. Any ideas?