Camry'98 question

I have a weird problem with my car. Sometimes it would start in the morning and immediately stop or would run a low RPMs(for a cold engine.)The car rattles, kind off, and does not move readily if I put in Reverse. I also noticed that the oil alarm light stays ON longer than usual and eventually goes off. Interestingly enough,if I step on the gas pedal right after it starts-->no problem: engine goes to higher RPMs like it should. Note: I've noticed that it happens when it's humid outside (muggy in the summer or rainy in the fall). As long as it's not humid (even when its very cold in the morning) problem does not manifest itself. I've replaced fuel pump, air filter, fuel filter, spark plugs, engine coolant temp. sensor - problem is still there. Fuel pressure is normal. Engine idle RPM are about 750. Please respond Thanx a million