How's the exhaust look? Exhaust tells a lot of stories. Exhaust at cold idle, warm idle, WOT, etc.
It's not the glow plugs, that much is certain.
It's probably the IP, but you could have other contributing factors too: weak compression, air intrusion, weak starter. Batteries/stock alt getting tired if you're only doing a 15 minute trip, it's probably not enough time to recover the power from GP and starter.
Blue exhaust means you're burning oil, compression is low. Gray at cold start is normal. Black when working hard is normal. Anything else could be abnormal.
My old engine was critically low on compression. The prior owner sucked a bunch of sand right before selling it to me, so the compression went south fast. I had to glow EVERY time I went to start it, unless it was under probably 10-15 minutes. It started fine in the summer but boy it hated winter, I needed the batteries in tip top shape, good new starter, and fresh plugs (bosch, which I felt ran hotter) on a manual button. Then it did OK, but if any of those qualifiers slipped up, like the batteries not being topped up, or 2+ dead plugs and she was not moving without a pull-start. Ran great when it was started, being turbo and all. Just a lot of blue, oh and it ran away once but being a manual you just upshift hard to ***** it. If it's an auto and she runs away, good luck. Assuming non-turbo Jam towels down the intake if you can get to it before it blows, if turbo get a thick piece of wood over the intake, just not your flesh, it's basically a garbage disposal from hell.
In fact I know the bosch glowplugs run hotter, I could manually hit them for 7 seconds and start up, with the Motorcraft it took much longer. But you can't deviate if you use the glow plug controller , it'll murder any hot plug - only Motorcraft beru or 12V-proper plugs can take that caveman technology abuse.