I had the same problem with my Spider Valve 112 (Mk 1). Bought it new a couple of years ago and as soon as the warranty expired, the problems started. The preamp shuts down and restarts after a couple of seconds then shuts down again and so on. The shop said that for a wagon of money they would replace the preamp = "Buy a new amp, sucker!"

After some troubleshooting, I found out that there is a design issue on the preamp circuit board. A voltage regulator gets overheated and its thermal protection shuts it down. The cut-outs was more frequent if I engaged Phaser, Echo and such effects which probably increases the load on the regulator. Regulator surface temp was measured to approx. 60 degrees C and since there is no other cooling than the printed circuit board (which is not a good heat conductor) to lead the heat away from the regulator, its junction temp will reach critical level when loaded.

It's not a big deal to fix this problem without that wagon of money. I just added a better cooling to the regulator and now it's an amp again.

A word of caution, though: If you're not handy with soldering and electronic repair jobs, I suggest you look up someone who is. It's about a half-an-hour job for a skilled electronic technician and the component cost will probably drain your wallet for about a dollar or so. If you like to see what it's all about, here are some pictures you can look at;