Jeep: 90 cherokee 4wd 6cyl 4.0L, compression issue, jeep wont start

Question
On my jeep i have replaced the ignition coil, spark plugs, fuel filter, distributor pick up unit, cleaned and tightened the grounds, and still sometimes my jeep wont start like it has no spark. I'm not sure if its just coincedence, but when I take off the air fuel canister lid it seems to start - hasnt always worked though. We are stumped, any idea?

Answer
I think you have a compression issue.  If your Jeep has over 100K on it, have the compression checked on each cylinder, if within reason, you might have other problems but my guess is that your compression is low across the board and creating the problems.