The number one cause of insufficient heat problems with dryers is poor air flow. The first thing you need to do BEFORE assuming you have any defective components is to do a full inspection of the dyer to ensure it is not clogged anywhere. The easiest way to determine if you have a dryer clog is to leave the vent hose removed from the back of the dryer and attempt to dry a load. The air flow leaving the dryer should be forceful and about 140 degrees F. If the air flow is weak, you more than likely have a clog in the air blower assembly INTERNAL to your dryer. If the air flow is normal and warm as it is should be, you may have a clog in the vent ducting EXTERNAL to the dryer. This would be from where the ducting leaves the dryer to where it exits your home. If you have a vent line that runs through and attic or crawl space, they are notorious for clogging. If you haven't had the ducting cleaned or inspected recently, now would be a good time. You should also be using the semi-rigid (metal type) ducting. It resists crushing, kinking, is heat resistant, and resists rodents chewing through it. Another concern I have is that you mentioned the dryer was in storage. It is common for rodents to build nests inside the blower fan housing area and clog the dryer. Since you mentioned your dryer is getting warm, but not drying like it should, tells me the heating element is working. You may be right and you could have an operating thermostat that is failing and not allowing the heating element to stay on long enough to heat properly. However, I would recommend you inspect the dryer thoroughly first to rule out poor ventilation. The following link expains some of what I've mentioned in greater detail:
I hope you find this information helpful. Let me know if you require further assistance.