Problem!! accelaration cut out

[miggysanch3z]

ok i have a problem here i got a 1990 240sx sohc millage is unknownthe problem is when i floor it 1st threw 3rd gear is fine but when i engage 4th and 4th gear reaches around 5 thousand rpms it decelerates by its self i step on the gas and no go. i have to put it to neutral and pop it back into 4thwhat could be causing this?

[the converted]

Start with a tune up (plugs, wires, check timing, and tps sensor) and go from there.