Model: 1985 745t, b230ft

Problem:
My engine is making a little noise, so I popped the hood for a closer inspection after a brief warm-up drive. As I was probing around I happened to place my finger on the rear (nearest to firewall) plug cap, and could feel an irregular knock/hit sensation. It's hard to describe, but it felt like something was tapping against the plug at regular cycles. When I felt the other plug caps the middle two were fine and felt smooth with none of this wierd slapping type effect. However, the front one (nearest radiator) did have a lesser version of the same problem as the rear. A last note to make is that when I push hard on the top of the rear plug cap the engine seems to stall a little bit (but it doesn't approach a stall out or anything).

Question:
What is this problem indicative of? Am I just detecting normal old volvo red block slapping or is this more serious? Is the fact the front and rear plugs display this effect indicative of a larger problem that I'm missing (i.e., are the front and rear tied together, and the middle two tied together)? I pulled the plugs about 3 months ago and they looked fine, and the caps are fairly snug as far as I can tell.

Sorry for my inadequacy in naming the plug/injectors by number, but I don't know what the order is or how to refer to them specifically.

Thanks in advance for any help the combind minds of the brickboard can offer.

-Al