My frame rate is good but when I hit the space bar the frame rate becomes weird.

When you say your older projects worked in the past, how long ago was this?  Both AE and your computer's OS may have been updated since and that generally means system resources are more spread out.

As Rick was saying, AE builds each frame into your computer's RAM before playing back.  It's got really good at reducing the time it takes to build up these frames.  You used to have to watch it one frame at a time, seeing mistakes frozen in place.  Now it's a lot quicker, but  part of the innovation with AE is that it can dump the RAM-previewed frames onto disc storage too, so when it plays, it is reading your drive, reading your RAM, keeping any audio synced up - there's a lot going on.  Keeping your cache clear and your graphics card drivers up to date can help.

After Effects relies on available system resources to render a preview before it can play back smoothly. The length of the preview depends entirely on the available resources. You need a really powerful system with a lot of free resources to ram preview more than about 7-seconds of a full HD comp or 4 seconds of a 4K comp. There may be nothing wrong except your expectations and workflow.