99 dodge neon needs help please?

So i had my oil changed two weeks ago at that visit i had them look at the a/c it had not been blowing cold. they said it was bc the free on was low it took 2lbs they said. No big deal i have also had issues with my speedometer i would be driving and it would go from 20 down to zero at the drop of a hat and bounce around and then eventually stay at zero or all the gagues would stop while i was driving lol. the mechanic said it was my computer and i needed to take it to the dealer. SO after my oil change and freeon was put in, my car gauge started to run hot i would pull over no smoke no nothing. it keeps doing it every time the ac is on or off. i dont know if this has somehting to do with the computer or gaugues or if there is a serious problem else where. my car will say its running hot and i will keep driving it and nothing happens but im getting worried. just looking for some advice before i drop money on this thing. thanks