New Car and New Problems...Help!!!

[my23car1]

In December of last year i purchased a 99 Alero with the 3400 engine... It ran great for a month or two and then hell brokke loose in the form of several little problems. I guess illl just start from the beginning...

1. The first of the issues occurs when i first start the car. Some times it will start right up, runs at the right rpm, and i have no problem. Other times, more frequent, it will start and jump back and forth between about 500 and 1100 rpms. I don't know what it is and its really becoming a problem.

2. The second happens when i am driving. This usually happens anytime i have been driving for a while, like 20 min or so. The temperature guage will drop as if it is not functioning and at the same time the RPMs will spike. When it is in drive at a stop it will be at a steady 1500 rpms, and if you shift it into park or nuetral it will scream at about 3000 to 3500.

I took it to a shop and they said they could find out what it was for about 50 dollars to run a diagnostic but im pressed for cash and i am mechanically able if i could just know what to do. I replaced the plugs and wires, regular oil changes, tranny fluid change when i got the car...just got it up to date. If anyone knows my problem, please help... I am new to aleros, always been with a grand am 2.4, so this is a little new to me. I plan to "pimp" it out performance wise in the future but i need to get it running okay first....thank you for any advise, Caleb