Fuzzy Sound in my Audio Research SP16 Phonostage

over the last week or so i have been having trouble with the phono stage in my Audio Research SP16 pre amp. for the first 15 or 20 minutes of listening to vinyl the sound quality is all fuzzy and staticy - like poor reception on a radio tuner. you can hear the music, but it is buried beneath a layer of distortion. over time it gradually goes way - after 5 minutes or so it starts to diminish, and it is generally completely gone in 15 minutes or so. it has me concerned.
this only happens on the phono stage - not on any other input. it does not matter how long i let the pre amp warm up either. if i turn on the pre amp and listen to vinyl right away, or if i listen to CDs for an hour and then switch to vinyl i still get the same fuzz.

by the way, my turntable is a rega P5 with an Exact II cartridge. as far as i can tell there does not seem to by anything wrong with the turntable. i have been using this turntable and cartridge with the SP16 for several months now, but this problem has only presented itself in the last week or so.

my guess is that i have a bad tube in the pre amp, but i really don't know for sure. anyone have advice for me?