My Hyundia Elantra after sitting for a long especially in the morning has trouble accelerating. l push the gas pedal all the way down to the floor and the car barely goes until all of a sudden the car takes off. This only happens in the summer time. Is humidity the problem?

First ? is the has the check engine light ever come on even if u did not see it have the system checked for DTC is (data trouble codes) it sounds to me like a bad TPS (throttle position sensor), it could also need a tune up. Whats the mileage if it is anything past 50k l strongly rec. a tune up. Also another ? year of the vehicle & mileage u might still be under warranty if ur the original owner. It would be a good idea to take it to a hyundai dealership there might be recalls they could perform for free or even downloads for the ECM or PCM & other systems. Hope this info was useful.

Transmission slipping? Bad fuel? Is the check engine light on?

Based on what you said, all will basically be guessing. Take it to a tech that knows what he is doing, have him evaluate the car. Obviously if the problem is in the morning and you do not have a check engine light (MIL) on, then you need to leave it overnight.

l agree with the last answer. It does sound like either an igniton related problem, or a possible restriction in the exhaust somewhere, but if it is a wet problem, l would lean heavily towards the wires and the ignition coil, as Hyundai has been famous for(among other things)bad ignition coils. When replacing, always stay with the OEM coil. More money, but far more reliable, and better warranty.