1998 Mitsubishi Eclipse Engine not starting

Brand new battery, alternator tested and works, starter tested and works. When I first put the new battery in the car I also replaced the positive cable end by cutting the original off and replacing it. So there is a new and solid connection to the positive terminal. At first, it started quickly, but within a few weeks it slowly and increasingly took longer to start. Thinking there might be a problem with the battery, I had it tested at two different auto stores (Pepboys, O'reilys). Both said the battery, only 2 months old was in perfect shape. Fully charged. A store employee (one who sold me the battery) suggested that there might be another issue like a switch or sensor. I was also told that the battery would recharge on a 20 minute drive if the alternator was working. So, I take it for a 30 minute drive, stop and turn it off, try starting it, but to no avail. It was starting at moderate outside temps: 60, 70's, but 90's 100's would not. I also have a radiator leak so I'll refill the radiator at destinations. Now I have to push start everywhere to get it going. Fortunately it's a manual trans w/clutch. Is it possible a sensor or electronic switch has gone bad? Thanks