Starting problem

I have a 98 S-10, 4cyl auto, 130K.
About once every 15 or 20 times I go to start the truck, it doesn't turn over normally.
Turns over very slow at first and then fast enough to start. Feels like it's going to start clicking like when you have an almost dead battery, but never does that. Charges normally.
It always starts. If I let off the key when it happens, it will usually turn over normally the next time I turn the key. Has new battery, so I replaced the starter and it does the exact same thing.
Guessing I may have a bad ground somewhere?

Most likely a poor connection. Check both ends of both battery cables: take them apart and clean them with a file or sandpaper, then tighten them up and you should be good to go.

You can also test the cables with a voltmeter by doing a voltage drop. Basically put the test leads at each end of the positive cable, preferably on the battery post and the starter solenoid positive battery terminal and then have someone crank the engine. You are measuring the amount of voltage it takes to go from the battery to the starter, so whatever the reading is, subtract that from 12 and that is how much is getting to your starter. You can do the same thing for the negative also.

still having the same problem. Does it more often now though. Cleaned the cable ends with no better result.
Any other ideas?

Check voltage drop as RahX described on the positive and the negative cables.