Volume Problem in Effects Loop

[shredmonster]

I have an Ibanez SDR 1000 rack unit hooked into the effects loop of my Framus Cobra.

After about 15 minutes the volume of my rig increases as does the wet signal.

I am wondering if my effects unit is doing this or if there could be something wrong with my effects loop itself. I had the amp in about a year ago and they replaced a fuse and rebiased it so I figured everything should have checked out OK with the amp but they could have missed something.

Then again the effects unit is an old unit from the 80's.

[Wayniac]

If it happens over time, it sounds like a heat issue to me - like after components get to a certain temp. That could be tubes (is it a tube FX loop) or it could be the FX unit itself... only a guess though...