I bought an inverter just like yours from Costco for $70. Wish I'd known they dropped in price; would have gotten another one. The instructions say to only install it flat. Can be upside down, but must be flat. Has to do with proper cooling by the little fan. My computer science son says the little fans used in electronics only meet minimum cooling needs. Could be the angle of the inverter doesn't let the fan do a good job. The Canadian company that made the unit has good customer service help. Call them and let us know what they think.
My 1000 watt inverter has been great. We run a 700 watt microwave, 9"color TV/VCR, DVD player, CD/radio and small refrigerator. We have two 110 amp sealed batteries. The inverter must not be more than 5 feet from the battery(s) and needs a 200 amp T fuse (cost more than the inverter). Be sure the cables to the inverter are 2 AWG. If lighter weight they will get very hot and not get enough voltage/amperage to the inverter.