Help! Strange tach behavior. Suddenly runs badly.

[dynamohum]

I have a 1994 Villager, and suddenly it's started to run badly.

Car ran beautifully before this problem occurred. The Check Engine light came on. Suddenly, the engine has lost power. Misses at an idle. Backfires when I rev it up. Strangest of all, the tachometer either sits at 1000 RPM (at an idle), or immediately jumps to 4000 RPM and sits there at any engine speed off of an idle. Has anyone experienced something like this before? Would appreciate any ideas you might have. Thanks! Mike.

History:
1. Check Engine light has been on since 47,000 miles, but he engine ran well. Found and replaced a piece of rotten vacuum line. The Check Engine light cleared itself after the 4th start. Hurrah!
2. Valve cover seals leaking, oil all over the engine. Engine was building up pressure, and blowing oil out the seams. Replaced the PCV. No improvement. Replaced three large rubber hoses - they were plugged. Cleared the problem. Hurrah!