You need to do a voltage drop test on the starter system to find if there's a bad connection. Start by measuring the battery voltage right on the posts, not the cable clamps. Turn on the head lights so some current is trying to flow. You will find 12.6 volts if the battery is fully charged. Next, move the meter probes to the cable clamps. The voltage must be the same. If it is lower one of those connections is loose or dirty. Keep moving the probes down the cables and measure at each connection. The last connection on the positive side is at the starter terminal. Ford has had a lot of trouble with the wire strands corroding away under the insulation where you can't see it, right next to where it's bolted to the starter terminal.
Normal voltage drop testing involves having a a helper try to crank the engine so you can take voltage readings while current is flowing in the circuit. That will identify the cause of slow cranking. All you said is you have a starting problem but you didn't say what that problem is. If you have a no-crank condition regular voltage tests will find the cause.