I have also had this happen recently - it looks like the raiders are sitting on top of their invisible jump point and hopping back and forth across it every 5 seconds. IIRC this behavior was fixed in 2.0 for regular NPRs on normal jump points, but it may/must still be broken for raiders.
Beam combat should be able to counteract this, especially using the "fire at will" function, which reduces fire delays. Doing this my fleet destroyed one raider ship and then the rest left for good.