Premiere playback freezes for 15 - 30 seconds when moving playhead

Hi Everyone

I really hope some one can help me with this it's making Premeire impossible to use and I need to get this feature film finished.

When I move the blue play head along the time line (there is no longr any scrubbing) from one place to another and hit the space bar,

if I'm lucky playback will start within 5 to 10 seconds, sometimes 15 - 30 and often just locks up and has to be restarted.

It has got a lot worse as we've been adding foley and ambient sound tracks, there are now 11 audio tracks and 12 video but there are

seldom more than 4 tracks being used at any one time.

The film “Axman” is 90 minutes long, does not have any special effects and I don't think I'm using any plug ins

I regularly use a memory cleaner which shows around 14gb free (it varies and I run the cleaner if it goes below 10gb)

All other programmes are shut down

99% of the footage is 1080p with only a couple of very short clips being 4k scaled to fit 1080p mainly MXF files 24 fps (but everything seems

to have fallen apart with the sound editing.

Have changed the media playback from Mercury playback engine to each of the other settings.

I have spent a lot of time sending details to Adobe tech people but not had any solutions offered

The main machine is :

iMac 27” 5k Retina

32gb RAM

4 tb Lacie external drive on thunderbolt

Other thunderbolt port used for HDMI out to a TV as second monitor

I have also tried it on a

Model Name: MacBook Pro

Model Identifier: MacBookPro15,1

Processor Name: Intel Core i7

Processor Speed: 2.6 GHz

Number of Processors: 1

Total Number of Cores: 6

L2 Cache (per Core): 256 KB

L3 Cache: 9 MB

Memory: 16 GB

Many thanks for any advice

Duncan