Extruder heat would go up with printing with heatbed

I have face a weird issue, when i was printing my extruder temperature will drop to 215-219 as a preset was 235, but when i never turn on my heatbed, my extruder can go up to 235 & higher, I have put a relay on my heatbed but still having this issue, should i add relay to extruder or maybe have a spoilt somethings, anyone can advise???? sincerely appreciate any kind help......

Depending on the firmware you are using, you can try to autoPID the extruder heater.
Or maybe your wires from PSU to controller are to thin and cause voltage drop.
Another issue might be, the FET for the extruder is not fully ON, because the 5V rail is somewhere around 4.5V-4.7V. ( Happens mainly on Arduino Mega with Ramps and graphic LCD )

o_lamp, can i know for FET for the extruder how do i do the setting ??? as i'm using ramp 1.4, mega 2560 + graphic LCD

I find out one issue and this solve the problem which is as attach. now my extruder able to go up to 240 without issue