Wierd intermittent problem.

[Organtis]

So one of my amps has a preamp issue. Sometimes it works fine and then 5 minutes later it starts cutting out. After that it seems that if I plug the guitar lead in it will make sound for a second and then go dead. If I continue plugging and unplugging the guitar lead it does the same thing.

Is this a capacitor? Which one.... This is a Randall RG75.

[GGBB]

Sounds more like a dirty/bad connection or bad solder joint somewhere that gets worse as the amp heats up and is affected by vibration like when something is jostled. Could be a bad cap though. Use an audio probe to trace the signal through the amp and find out where it dies.

[Fender3D]

more likely is a bad solder joint...
time to put in use your favorite magnifying glass...

Edit:
GGBB beat me to it

[GibsonGM]

YUP!  If you can safely open it up to expose the PCB, you could run a tone generator in there, let it warm up til it starts doing it...then, you gently tap around w/the eraser of a pencil to try to find where just a tiny bit of flex causes/cures the problem.   Inspect, probably a solder joint.

Be careful, tho - you must understand what not to touch if you do this! 

[Fender3D]

If it is like most amps I repaired, you have issues with send/ret jacks...

[Quackzed]

if it is the send/return jacks (my money is on 'em) you can stick a plug in each one and wiggle it a bit, when those jacks are unused they get corroded and the connection (when nothing is plugged in em) gets ...well oxidized and doesnt make good connection... so sticking a plug in each and wiggling it can SOMETIMES rub off the oxidation on the connection pin and it'll make batter contact. itll probably go bad again without getting in there and cleaning the contacts, but it should give you an idea if thats the issue, and temporariy fix it, ish...
wiggle the plug or better, put the plug in / out a bunch of times... (your trying to use the side of the plugs tip to rub off the 'um... oxidation.) try to find a mental metaphor as you do it...   

[robthequiet]

If I continue plugging and unplugging the guitar lead it does the same thing.

My MESA manual says take a patch cable and connect the effect out to the effect in. That way you can wiggle the one cable and get an idea if it's those jacks. First thing I would do if you haven't already, check the guitar input jack. With power off, give the wires a gentle yank [as long as you have the means to resolder, of course.]



Otherwise, all of the above 

Is this resolved yet? I had a little Fender Champ that needed a kick every so often, which worked but not recommended.