And / or ::
Slacken off motor bolts on motor 1( a turn or so - motor must not come loose so only quick bit of throttle to check ) .
If no improvement , tighten and proceed with same on motor 2 and so on.
If improvement seen , re mount motor bolts with washer under bolt heads. ( / remove mud )
An other quick way to test is to isolate the antenna from the frame of the craft - i.e. Props off ,unbolt the sma from frame , make sure vtx and pigtail and antenna aren’t touching frame - if improvement , then "further research " with multimeter/ circuit tester .