Question on old toyota camry alternator?

I have an 88camry. the alternator recently pooped on me. as it was going, when I would put in in gear, or turn on the AC, the engine would slow down to such a slow idle it would almost die, then it did die. they told me that this is because it was drawing on the alternator and the alternator just didn't have enough oomph to keep the car going. so I got a new alternator. first few weeks it ran great, but now,whenever I put it in gear, it runs very , very slowly,the RPM's are like 300, barely enough to keep it running. and this is without the AC on. it acts like it's about to die at any second, just like it did before replacing the alternator. but you would think that the mechanic would have tested the entire system to see if it was something else. also it has had a recent tune up with new plugs and wires and rotor, etc. it's a fuel injected car. can anything else be going on here? the car does have 235,000 miles on it and usually runs so quiet it's like an electric car, it just purrs, but now it acts like it want's to die again when you put it in gear, and it did actually die once while in gear. the RPMS are sooo low, just barely enough to keep it running while in gear.