Starrting Probs

Having probs with my 97 GT . It sat fir about 5 months while I worked on it. Battery was dead so I tried jumping it...No go.
Bought a new battery and wham started on first crank..Let it run for abbout 15 mins shut it off. on saturday night. Sunday late morning started it went and got tires and aligment. Started fine both times brought it home shut it off. Monday morning turn the key and nothing. try jumping ...No go.
Take the battery out let it slow charge all day. Tues morning go out put battery in start it up no probs go to work unhook positive terminal thinking I have a short . After work hook up terminal starts fine. Get home unhook cable again. Weds and thursday morning same thing. Thursday nite get home leave cable hooked for 30-45 mins. Go out start to car to run quick errand starts fine. 5 mins to pick up some paperwork go to start car...nothing dead as all get out. Have friend come to jump me..hook up cables get power turn key ..click then nothing...try it agin..same
Thy a third time same then no power at all. Take terminal off battery hook jumper to cable power back but still will not start...

Puzzled at it all any suggestion?????

Welcome to AF!!

It appears that you may have issues with your main battery cables. They will corrode over time and nothing will be visible externally. Also, you need to look for a short or a light that is not going out when the car is shut off. Something is draining the battery and causing these issues. Jumping a vehicle will help a weak battery but if it is totally dead a jump may not allow the car to start. Also, you may want to check the connections to the starter solenoid and the ground wire on the block. Since the car sat for 5 months they may be corroded as well.

Thanks will tryall of that. Funny how it just crops up. Will start inside as the stero is aftermarket and I did not install it. Wanna see what previous owner did.
Don't think it would be from the work I did as it was all front end but ya never know