I have a 2005 Hyundai Elantra which just started jumping hard into 2nd and 3rd gears especially is I am accelerating it does have over 100 thousand miles but I have kept up with the oil changes pretty regularly I have not done it however for the last 3 thousand mile visit. Could this be the problem? It's really clunking sometimes.

Clutch is slipping.

Governor pressure controls the 1st and 2nd shifts. After that is the 3 and 4 solenoids. you would need to look at the pressure when driving(takes a scanner). pressure is 20 to 30 PSI from 1st to 2nd, depends on acceleration. After that the pressure will jump to 100+ psi. Did you replace the governor solenoid and sensor? They are on the passenger side of the transmission and sit in a block that detaches from the valve body.

try gear oil level / selectors / thrust bearing / presure plate

Look at the modulator on the side of the transmission. It's probably gone bad.

probably the gear is wearing pout

Check sprak plugs to start. Sounds like possible cylinder not firing.

First thing ... pull the dipstick and check the oil. It should be bright pink or red, if it's brown or black and smells bad your oil has never been changed. Drain as much as you can, measure how much came out and replace it with synthetic compatible oil. Start the car and let is run for a few minutes and shift the gears a few times ... then take it for about a 10 mile ride to circulate the oil, stop and go now and then so it will shift through all the gears. You could also add about half a quart of Lucas Transmission Fix.

sounds like the symptom of a bad clutch, might need adjustment at local repair service.

Throwout bearing will need to be replaced most likely. Otherwise cluctch plate or fork may have stuck

balls