One way to make sure your timing is close enough, is to take the number 1 plug out and with a compression gauge (or the old thumb) run it up to TDC compression. I use a slightly bent piece of welding rod to make sure I´m at the top. Check and see how close your timing mark is, then look and see how close your rotor is to number one on the cap. It doesn´t have to be exactly at the "1" on the cap but close.
Make sure you are at TDC compression, before changing anything. The exhaust stroke can build a little pressure that may fool you.
When your sure, the motor is at "1" TDC compression, look at your number one rockers, you should be able to wiggle the rocker arms a little and they should be close to level with each other. There is very little tension on the "1" rockers, when the motor is at TDC "1".
Which 2.5 do you have?
I had what I originaly thought was a timing problem, turned out to be a bent valve. When I was sure the ignition was in order, you could see it with a vacuum gauge, needle swung pretty far at a regular interval. Did have one cylinder with a slightly low compression, but the valve never did completely unseal.
I ran each cylinder up to TDC (top of the compression stroke) and I could see that the rocker arms on the affected cylinder, weren´t level and/or looked different than the others.