Hi. Having some trouble with a 2007 fg25t-16. Serial 210459a. The machine hasn't been used much in the last 2 years. Less then once a month to move items. The machine has 3000 hours. Recently we used it to pick up some heavy loads that weighted 5000 pounds. The machine picked it up just fine and put it down just fine. However when you would stack items and pull out with the forks unloaded and lowered them the forks would stop about 10-12 inches above the ground. If someone would stand on the forks (approx 160 lbs) and you lifted the forks up a few inches and lowered them, they would go back down to the ground as normal. This seems to only happen after picking up a heavy load. If I pick up a 1500 skid and stack it then lower the forks unloaded they will travel completely down.
I checked the rollers in the mast and they all seem to be spinning. Sprayed wd40 for good measure. Didn't notice any binding. It almost seems like the pressure is being relieved and without a little bit of more weight the forks stop.

I'm not sure if this is relevant at all but this machine came from a rental fleet. When I purchased it one of the chains was off. Not sure how it slipped off but I loosened the 2 nuts on the anchor bolt put the chain back on and tightened the nuts back up. The chains have grease on them and the forklift is stored outdoors with the cab covered.

Looking for any insight into this problem. This is what I'm thinking. Start with the simple things first. I do not know when the hydraulic fluid and filter were last replaced. The fluid looks golden when checking the measuring stick but maybe stuff has settled on the bottom. What else can I check or what should I do next?

Thanks in advance for the help