oil is new and full,during idle oilpressure drops and engine idles rough, stalls

My 1992 Buick Roadmaster Estate wagon has 50,000miles, has a slight idle tremor while stopped.At first it would idle under 500 rpm's and the oil pressure would drop below 20, then eventually stall, while I had my foot on the brake. If I placed in neutral the idle would be a bit better, but could still feel the tremor. I replaced old sparks with new, the tremor is better but noticeable.Could it be the timing chain?, what is the cost of repair?

It sounds like you could have a vaccume leak in the brake booster sys.