Huges and kettner vortex 80 problem

[Deegatron]

Heya Guy's,
Im looking for a little bit of trouble shooting help.  I've got an intermittant problem with my hughes and kettner vortex 80 head.  I've tested the cab speaker cable and the guitar cable with another amp and they check out 100% O.K.
once in a blue moon she'll just randomly drop 60 percent of the volume and get fuzzy... it seems to happen more often when she's been running for a little while but im pulling my hair out trying to find the problem....
Some days she wont play right all day... other days (like today) i cant get her to do it more than once...
2 or 3 times already i've thought i had the problem figgured out.. only to reassemble the head and then have the problem reappear....  putting the chassis back into the head cabinet does it every time... im assuming it's heat related as the cabinet is not exactly well ventilated....?
Used the line out direct to headphones and she sounds fantastic even when the sound through the cab sounds like crap...
I suspect the problem is in the power amp.
I found the complete schematic here
http://www.freeinfosociety.com/electronics/schematics/audio/hughesandkettnervortex80.pdf
I've pulled the head apart... nothings burnt... none of the solder joints look suspect to me.. everything looks fine....
tested the power and it checks out just like it should...
mains come in at 120
comes of the transfermer at just over 30 and after the rectifier diodes she's running a solid 41-0- -41 volts....
just like it should be.....
sadly... that's about wher my technical knowledge leaves me....
any help finding the problem or trouble shooting techniques would be muchly appreciated.
Thanx,
Jon

[DJPhil]

Given that your problem is so frustratingly intermittent, my first instinct is mechanical. It's probably because I just had a horrible run with a headphone jack myself.

In my case the jack's innards were exposed, so I could cram a wooden toothpick between the switching contacts. This verified in my case that one of the contact arms had lost it's spring, so it was intermittently open circuiting the output. A quick cleaning and a pinch with the pliers solved it for me.

Hope that helps.

[Deegatron]

You sir.... are an absolute genius......
a patch chord between my fx send and fx return... instantly fixes the amp... when the patch chord is removed... the amp is magicly... re broken.....
I cant belive i've been battling this thing for over a month now.... and have thought about posting here repeatedly and never..... gotten around to it.
I bow to you're wisdom.....
now what the heck am i gonna do with the spare parts i ordered.......
crapola.... I'll just have to build a tilt-a-whirl.....