Hey dneu (btw, we use our real names here, please read the Forum Rules),
With any new (to you) machine, it's important to thoroughly clean the tape path. That includes the heads and any guides that the tape comes in contact with. Using a good light and some sort magnification helps.
Also, the tape(s) you're trying to play may be sticky. Particularly if they're Ampex 456 or other back coated tapes of that era.
You should get some new tape and designate a reel just for testing and working on the machine.
The 2 current production tapes can be found here;
http://usrecordingmedia.com/Another fairly common issue that leads to erratic tape speed are binding brakes. Take a look at your service manual and perform the brake tests and if needed, adjust the brakes.
One pitfall that many machine owners fall into is adjusting things without knowing that they do indeed, require adjustment.
So, if you're not sure, don't adjust it! Some adjustments require special gear to adjust properly and if you don't have that gear, you're bound to make things worse.
Unless you have a good bit of experience with the machine, it's always a good idea to take it to a good service tech. He/she can go through the machine and lubricate, clean and adjust things to factory spec. They can also give you an idea if the heads need lapping which ideally should be done before all the fine adjustments.
Getting everything checked over and adjusted by an experienced tech will give you a baseline so you'll know when something isn't right.
If you're new to the wonderful world of reel to reel tape, take a gander at the "Beginners Guide" located in a stick above the General Forum.
