Sadly, the TGT IR in F-15 did NOT have a Ground Stabilized mode. Was my pet peeve from the get go...and once implemented in F/A-18, it was hard to go back to F-15. The only workaround was to break immersion and PAUSE the sim, which allowed you to fine tune the TGT IR without the SNOWPLOW mode ruining things.
Only real non-cheat workaround was to really concentrate on a good HRM and manage to reset the TGT precisely enough so that slaving the TGT IR to the TGT point was enough..
Back in the DAY, Starbuck wrote a Training mission with a method to accomplish this without pausing... I have not flown it recently, so I cannot comment, but here it is if you want to try it...
Starbuck AGTrain3.zip