trany shifts to fast

hi every one have 92 gmc 5.7 170000 on it the transmision shifts to fast to overdrive and my guages flicker when any power is turned on raido cuts out when it warms up has a surge driveing about 35 40 mph things have done to the truck new gound battery cable ,dis and cap rotor, plugs, wires, egr valve ,coil ,tps ,speed sensor new fuel pump and sending unit mass air flow sensor, o2 sensor, changed tranny filter ,coolent sensor the only thing i can think of to change is the ecm or cat is plugging up when it warms up any help thanks for your time:banghead: