97' accelation issue

Here is the problem. My 97 k1500 5.0L feels sluggish. 154000miles. Has the feeling of almost towing something under normal acceleration. If I stomp on it, it will downshift and go no problem, but normal situations is when I seem to have the issue. Have changed plugs, wires, rotor, cap, pcv valve, fuel filter, and had fuel system professional flushed (extra 40miles to the tank now). It does have a harsh 1-2 2-3 shift sometimes but the tranny shop found no damage and says it looks fine. Went ahead and serviced that anyway. Still feels kinda gutless. Does not appear to leak anything. Do lose about 2/3rds quart oil every 3 months. Coolant seems a little bit lower then I remember, not much though. Also, it does have a 4" suppension lift with 33x12.5's but, did not have this kind of power loss until recently. So, any suggestions?

Update

Think I fixed it. I decided to rip out the sparkplugs and put in the ACDelco's. Alot more power and throttle response. Had the older plugs in there for about 6 months. They were all in good condition so that is a good sign. They all had a bit of white deposits on them that I assume means that they were running cold. I will repost if anything else happens.

Update

Think I fixed it. I decided to rip out the sparkplugs and put in the ACDelco's. Alot more power and throttle response. Had the older plugs in there for about 6 months. They were all in good condition so that is a good sign. They all had a bit of white deposits on them that I assume means that they were running cold. I will repost if anything else happens.

What brand of plugs did you have in there for 6 months?

//2000CAYukon

Well, I had autolites. I had autolites in there before that too but, that set ran better then the most recent set. The recent ones started out ok, but performance was lacking over time. These ACDelco's are nice. I can really feel the difference. It even shifts smoother now. Why, couldn't tell ya.