I'm the new owner of an 88 with a 350 engine swap. I had an issue with the tach reading abnormally high (red line) RPMs, but the engine was not corresponding with these readings. The engine did, however, correspond with another issue. When driving on the highway and letting off the gas pedal, the RPMs on the tach fluctuate, and I can hear it in the engine.

Today I replaced the stock cluster with a new one and the high RPM issue is fixed, but the fluctuating issue is still there. When I let off the gas, it will be at 1.5k RPMs, and drop to 1k, with about 1 second intervals in between.

Any help with this issue would be helpful. Thank you in advance. Also, I did a search and couldn't find anything on this specific issue.