solid state amps howling

[acorkos]

i have here 2 solid state 2x12 amps....a marshall mg250dfx and crate xt120r. they both are working normally, it seems, except that after a few minutes (maybe after things warm up?) they start to howl/feedback at even low gain settings on their lead channels. it's weird that both are doing it, and i have worked on other similar amps and never experienced this kind of noise until the gain control is set much higher. any ideas as to what's gong on or where i should start looking for problems? thanks!

[Enzo]

So they do it with NOTHING plugged into the input jacks?

Does the reverb setting have any effect on the noise?